diff --git a/core/tests/Drupal/BuildTests/Composer/Template/ComposerProjectTemplatesTest.php b/core/tests/Drupal/BuildTests/Composer/Template/ComposerProjectTemplatesTest.php
index 641dba464b..1660cabbcb 100644
--- a/core/tests/Drupal/BuildTests/Composer/Template/ComposerProjectTemplatesTest.php
+++ b/core/tests/Drupal/BuildTests/Composer/Template/ComposerProjectTemplatesTest.php
@@ -112,6 +112,16 @@ public function testTemplateCreateProject($project, $package_dir, $docroot_dir)
 
     $this->copyCodebase();
 
+    // Keep our working directory around for inspection
+    $this->destroyBuild = FALSE;
+
+    // Make a working COMPOSER_HOME directory for setting global composer config
+    $composerHome = $this->getWorkspaceDirectory() . '/composer-home';
+    mkdir($composerHome);
+
+    // Disable packagist
+    $this->executeCommand("COMPOSER_HOME=$composerHome composer config -n -g repo.packagist false");
+
     // Get the Drupal core version branch. For instance, this should be
     // 8.9.x-dev for the 8.9.x branch.
     $core_version = Composer::drupalVersionBranch();
@@ -123,17 +133,19 @@ public function testTemplateCreateProject($project, $package_dir, $docroot_dir)
     // Always add drupal/core as a path repo.
     $path_repos['drupal/core'] = $this->getWorkspaceDirectory() . '/core';
     foreach ($path_repos as $name => $path) {
-      $this->executeCommand('composer config repositories.' . $name . ' path ' . $path, $package_dir);
+      $this->executeCommand("composer config -n repositories.$name path $path", $package_dir);
       $this->assertCommandSuccessful();
     }
 
     $repository_path = $this->getWorkspaceDirectory() . '/test_repository/packages.json';
     $this->makeTestPackage($repository_path, $core_version);
 
+    $this->executeCommand("composer config -n repositories.local composer file://" . dirname($repository_path), $package_dir);
+
     $autoloader = $this->getWorkspaceDirectory() . '/testproject' . $docroot_dir . '/autoload.php';
     $this->assertFileNotExists($autoloader);
 
-    $this->executeCommand("COMPOSER_CORE_VERSION=$core_version composer create-project $project testproject $core_version -s dev -vv --repository $repository_path");
+    $this->executeCommand("COMPOSER_HOME=$composerHome COMPOSER_CORE_VERSION=$core_version composer create-project --no-ansi $project testproject $core_version -s dev -vv --repository $repository_path");
     $this->assertCommandSuccessful();
 
     // Ensure we used the project from our codebase.
@@ -182,36 +194,64 @@ public function testTemplateCreateProject($project, $package_dir, $docroot_dir)
    *   The version under test.
    */
   protected function makeTestPackage($repository_path, $version) {
-    $json = <<<JSON
-{
-  "packages": {
-    "drupal/recommended-project": {
-      "$version": {
-        "name": "drupal/recommended-project",
-        "dist": {
-          "type": "path",
-          "url": "composer/Template/RecommendedProject"
-        },
-        "type": "project",
-        "version": "$version"
-      }
-    },
-    "drupal/legacy-project": {
-      "$version": {
-        "name": "drupal/legacy-project",
-        "dist": {
-          "type": "path",
-          "url": "composer/Template/LegacyProject"
-        },
-        "type": "project",
-        "version": "$version"
+    $root = $this->getDrupalRoot();
+
+    // Create a package definition for our template projects, metapackages,
+    // composer plugins and other path repository projects, e.g. drupal/core.
+    $packages = [
+      "packages" => [
+        "drupal/recommended-project" => [
+          "$version" => [
+            "name" => "drupal/recommended-project",
+            "dist" => [
+              "type" => "path",
+              "url" => "composer/Template/RecommendedProject"
+            ],
+            "type" => "project",
+            "version" => "$version"
+          ]
+        ],
+        "drupal/legacy-project" => [
+          "$version" => [
+            "name" => "drupal/legacy-project",
+            "dist" => [
+              "type" => "path",
+              "url" => "composer/Template/LegacyProject"
+            ],
+            "type" => "project",
+            "version" => "$version"
+          ]
+        ]
+      ]
+    ];
+
+    // Build out additional package definitions for everything installed in
+    // the vendor directory.
+    $installed = json_decode(file_get_contents($root . "/vendor/composer/installed.json"), TRUE);
+    foreach ($installed as $package) {
+      $name = $package['name'];
+      $path = "vendor/$name";
+      $projectVersion = $package['version'];
+      $projectType = $package['type'];
+      if (is_dir($path)) {
+        $packages[$name] = [
+          $projectVersion => [
+            "name" => $name,
+            "dist" => [
+              "type" => "path",
+              "url" => "$path"
+            ],
+            "type" => "$projectType",
+            "version" => "$projectVersion",
+          ],
+        ];
       }
     }
-  }
-}
-JSON;
+
+    $json = json_encode($packages);
     mkdir(dirname($repository_path));
     file_put_contents($repository_path, $json);
   }
 
 }
+