diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php b/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php
index d66d7802f9..2a75e795af 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Fixtures.php
@@ -167,7 +167,7 @@ public function sourcePath($project_name, $source) {
    * @see \Drupal\Component\Scaffold\ManageOptions::getLocationReplacements()
    */
   public function getLocationReplacements() {
-    $destinationTmpDir = $this->mkTmpDir();
+    $destinationTmpDir = $this->mkTmpDir('location-replacements');
     $interpolator = new Interpolator();
     $interpolator->setData(['web-root' => $destinationTmpDir, 'package-name' => 'fixtures/tmp-destination']);
     return $interpolator;
@@ -237,14 +237,14 @@ public function destinationPath($destination, Interpolator $interpolator = NULL,
   /**
    * Generates a path to a temporary location, but do not create the directory.
    *
-   * @param string $extraSalt
-   *   Extra characters to throw into the md5 to add to name.
+   * @param string $prefix
+   *   A prefix for the temporary directory name.
    *
    * @return string
    *   Path to temporary directory
    */
-  public function tmpDir($extraSalt = '') {
-    $tmpDir = sys_get_temp_dir() . '/composer-scaffold-test-' . md5($extraSalt . microtime());
+  public function tmpDir($prefix) {
+    $tmpDir = sys_get_temp_dir() . '/composer-scaffold/' . uniqid($prefix, TRUE);
     $this->tmpDirs[] = $tmpDir;
     return $tmpDir;
   }
@@ -252,14 +252,14 @@ public function tmpDir($extraSalt = '') {
   /**
    * Creates a temporary directory.
    *
-   * @param string $extraSalt
-   *   Extra characters to throw into the md5 to add to name.
+   * @param string $prefix
+   *   A prefix for the temporary directory name.
    *
    * @return string
    *   Path to temporary directory
    */
-  public function mkTmpDir($extraSalt = '') {
-    $tmpDir = $this->tmpDir($extraSalt);
+  public function mkTmpDir($prefix) {
+    $tmpDir = $this->tmpDir($prefix);
     $filesystem = new Filesystem();
     $filesystem->ensureDirectoryExists($tmpDir);
     return $tmpDir;
@@ -342,28 +342,21 @@ public function runScaffold($cwd) {
    *   The Composer command to execute (escaped as required)
    * @param string $cwd
    *   The current working directory to run the command from.
-   * @param int $expectedExitCode
-   *   The expected exit code; will throw if a different exit code is returned.
    *
    * @return string
    *   Standard output and standard error from the command.
    */
-  public function runComposer($cmd, $cwd, $expectedExitCode = 0) {
+  public function runComposer($cmd, $cwd) {
     chdir($cwd);
     $input = new StringInput($cmd);
     $output = new BufferedOutput();
     $application = new Application();
     $application->setAutoExit(FALSE);
-    try {
-      $exitCode = $application->run($input, $output);
-      if ($exitCode != $expectedExitCode) {
-        print "Command '{$cmd}' - Expected exit code: {$expectedExitCode}, actual exit code: {$exitCode}\n";
-      }
-    }
-    catch (\Exception $e) {
-      print "Exception: " . $e->getMessage() . "\n";
-    }
+    $exitCode = $application->run($input, $output);
     $output = $output->fetch();
+    if ($exitCode != 0) {
+      throw new \Exception("Fixtures::runComposer failed to set up fixtures.\n\nCommand: '{$cmd}'\nExit code: {$exitCode}\nOutput: \n\n$output");
+    }
     return $output;
   }
 
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php
index c27e3e2fdf..0c24d78cf0 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ManageGitIgnoreTest.php
@@ -169,6 +169,7 @@ public function testUnmanagedGitIgnoreWhenGitNotAvailable() {
     $this->assertFileNotExists($sut . '/docroot/sites/.gitignore');
     // Confirm that 'git' is available (n.b. if it were not, createSutWithGit()
     // would fail).
+    $output = '';
     exec('git --help', $output, $status);
     $this->assertEquals(0, $status);
     // Modify our $PATH so that it begins with a path that contains an
@@ -179,10 +180,38 @@ public function testUnmanagedGitIgnoreWhenGitNotAvailable() {
     chmod($unavailableGitPath . '/git', 0755);
     putenv('PATH=' . $unavailableGitPath . ':' . getenv('PATH'));
     // Confirm that 'git' is no longer available.
+    $output = '';
     exec('git --help', $output, $status);
     $this->assertEquals(127, $status);
     // Run the scaffold command.
+    $output = '';
     exec('composer composer:scaffold', $output, $status);
+    $expected = <<<EOT
+0
+
+Scaffolding files for fixtures/drupal-assets-fixture:
+  - Copy [web-root]/.csslintrc from assets/.csslintrc
+  - Copy [web-root]/.editorconfig from assets/.editorconfig
+  - Copy [web-root]/.eslintignore from assets/.eslintignore
+  - Copy [web-root]/.eslintrc.json from assets/.eslintrc.json
+  - Copy [web-root]/.gitattributes from assets/.gitattributes
+  - Copy [web-root]/.ht.router.php from assets/.ht.router.php
+  - Skip [web-root]/.htaccess: overridden in fixtures/drupal-composer-drupal-project
+  - Copy [web-root]/sites/default/default.services.yml from assets/default.services.yml
+  - Skip [web-root]/sites/default/default.settings.php: overridden in fixtures/scaffold-override-fixture
+  - Copy [web-root]/sites/example.settings.local.php from assets/example.settings.local.php
+  - Copy [web-root]/sites/example.sites.php from assets/example.sites.php
+  - Copy [web-root]/index.php from assets/index.php
+  - Skip [web-root]/robots.txt: overridden in fixtures/drupal-composer-drupal-project
+  - Copy [web-root]/update.php from assets/update.php
+  - Copy [web-root]/web.config from assets/web.config
+Scaffolding files for fixtures/scaffold-override-fixture:
+  - Copy [web-root]/sites/default/default.settings.php from assets/override-settings.php
+Scaffolding files for fixtures/drupal-composer-drupal-project:
+  - Skip [web-root]/.htaccess: disabled
+  - Copy [web-root]/robots.txt from assets/robots-default.txt
+EOT;
+    $this->assertEquals($expected, $status . "\n\n" . implode("\n", $output));
     $this->assertFileExists($sut . '/docroot/index.php');
     $this->assertFileNotExists($sut . '/docroot/sites/default/.gitignore');
   }