diff --git a/core/phpunit.xml.dist b/core/phpunit.xml.dist
index 2a55d680f3..1b0a01439f 100644
--- a/core/phpunit.xml.dist
+++ b/core/phpunit.xml.dist
@@ -26,6 +26,8 @@
     <env name="SIMPLETEST_BASE_URL" value=""/>
     <!-- Example SIMPLETEST_DB value: mysql://username:password@localhost/databasename#table_prefix -->
     <env name="SIMPLETEST_DB" value=""/>
+    <!-- Example SIMPLETEST_IGNORE_DIRECTORIES value: build,dev_module -->
+    <env name="SIMPLETEST_IGNORE_DIRECTORIES" value=""/>
     <!-- Example BROWSERTEST_OUTPUT_DIRECTORY value: /path/to/webroot/sites/simpletest/browser_output -->
     <env name="BROWSERTEST_OUTPUT_DIRECTORY" value=""/>
     <!-- To disable deprecation testing completely uncomment the next line. -->
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index 0a611d87c2..707e25c043 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -507,6 +507,12 @@ protected function initFileCache() {
    */
   private function getExtensionsForModules(array $modules) {
     $extensions = [];
+
+    // Set directories to be ignored during filesystem scanning.
+    $settings = Settings::getAll();
+    $settings['file_scan_ignore_directories'] = explode(',', getenv('SIMPLETEST_IGNORE_DIRECTORIES'));
+    new Settings($settings);
+
     $discovery = new ExtensionDiscovery($this->root);
     $discovery->setProfileDirectories([]);
     $list = $discovery->scan('module');
diff --git a/core/tests/bootstrap.php b/core/tests/bootstrap.php
index 7eb6ecb70c..a0fc1883b1 100644
--- a/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -22,7 +22,13 @@
  */
 function drupal_phpunit_find_extension_directories($scan_directory) {
   $extensions = [];
-  $dirs = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($scan_directory, \RecursiveDirectoryIterator::FOLLOW_SYMLINKS));
+
+  $ignore_directories = explode(',', getenv('SIMPLETEST_IGNORE_DIRECTORIES'));
+  $directory = new \RecursiveDirectoryIterator($scan_directory, \RecursiveDirectoryIterator::FOLLOW_SYMLINKS);
+  $filter = new \RecursiveCallbackFilterIterator($directory, function ($current, $key, $iterator) use ($ignore_directories) {
+    return !in_array($current->getFilename(), $ignore_directories);
+  });
+  $dirs = new \RecursiveIteratorIterator($filter);
   foreach ($dirs as $dir) {
     if (strpos($dir->getPathname(), '.info.yml') !== FALSE) {
       // Cut off ".info.yml" from the filename for use as the extension name. We