Use RecursiveExtensionFilterIterator in drupal_phpunit_find_extension_directories for performance

index 82a1830547..d956ff0e1f 100644
--- a/core/tests/bootstrap.php

--- a/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php

@@ -8,7 +8,9 @@
 use Drupal\Core\Composer\Composer;
+use Drupal\Core\Extension\Discovery\RecursiveExtensionFilterIterator;
 use PHPUnit\Runner\Version;

@@ -22,7 +24,9 @@
 function drupal_phpunit_find_extension_directories($scan_directory) {
   $extensions = [];
-  $dirs = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($scan_directory, \RecursiveDirectoryIterator::FOLLOW_SYMLINKS));
+  $filter = new RecursiveExtensionFilterIterator(new \RecursiveDirectoryIterator($scan_directory, \RecursiveDirectoryIterator::FOLLOW_SYMLINKS | \FilesystemIterator::CURRENT_AS_SELF), []);
+  $filter->acceptTests(TRUE);
+  $dirs = new \RecursiveIteratorIterator($filter);

This certainly brings phpunit a bit nearer to what \Drupal\simpletest\TestDiscovery::registerTestNamespaces is doing: Using our own extension discovery. I'm wondering conceptually, is it okay to have tests relying on code which will be tested by it? I think it is worth, given the huge speed difference! I think for running on the testbot it won't make a lot of difference right now, as this all still goes through run-tests.sh, but for the local development this method brings huge benefits.

@larowlan
It feels like this is not really a bug, rather a task, any reasoning for this?

1. +++ b/core/tests/bootstrap.php
   @@ -22,7 +24,9 @@
   -  $dirs = new \RecursiveIteratorIterator(new \RecursiveDirectoryIterator($scan_directory, \RecursiveDirectoryIterator::FOLLOW_SYMLINKS));
   +  $filter = new RecursiveExtensionFilterIterator(new \RecursiveDirectoryIterator($scan_directory, \RecursiveDirectoryIterator::FOLLOW_SYMLINKS | \FilesystemIterator::CURRENT_AS_SELF), []);
   

   Should there be a @see \Drupal\Core\Extension\ExtensionDiscovery::scanDirectory maybe? Is there a reason we don't pass along all the flags?

       $flags = \FilesystemIterator::UNIX_PATHS;
       $flags |= \FilesystemIterator::SKIP_DOTS;
       $flags |= \FilesystemIterator::FOLLOW_SYMLINKS;
       $flags |= \FilesystemIterator::CURRENT_AS_SELF;

2. +++ b/core/tests/bootstrap.php
   @@ -49,22 +53,16 @@ function drupal_phpunit_contrib_extension_directory_roots($root = NULL) {
   -    $paths[] = is_dir("$path/modules") ? realpath("$path/modules") : NULL;
   ...
   +    $paths[] = "$sites_path/$site";
   

   I wonder, do we need to call realpath on these files, like previously?

Was just scanning if we have any test coverage for this and looks like we don't, see comment in \Drupal\Tests\Core\Test\StubTestSuiteBase::findExtensionDirectories

So manual testing for this I guess

FYI, I am working on a patch in #2943172 that uses ExtensionDiscovery to discover test files.
This comment describes the motivation why ExtensionDiscovery should be used instead of RecursiveExtensionFilterIterator: https://www.drupal.org/project/drupal/issues/2943172#comment-13210705

Maybe this issue could be closed as a duplicate if you agree with the approach in the other issue.