Ensure that intentionally failed test stage directories are deleted after tests complete

I think this issue was postponed on #3328234: Improve test DX *and* confidence: stop using VFS which just got committed

#3315834: GitExcluder should not ignore .git directories that belong to packages installed by Composer was fixed🎉

Before starting with this I have a thought about how can we achieve this,
one way is to check each and every test and destroy the stage where it is not destroyed already.
the other way can be using tearDown() but I don't know how can we get the stage in that case?

As discussed in meet one ways to do this are,

1. In assertResults() we can add a finally block and delete stage in that
2. In teardown we can delete the stage directory.

question about the teardown approach, can we do that in the PackageManagerKernelTestBase and other testBases also?

#12
I think we want to go with 2)

but slightly different in tearDown() we should delete `\Drupal\package_manager\PathLocator::getStagingRoot()`
and \Drupal\package_manager\PathLocator::getProjectRoot() but we should only delete getProjectRoot() if is directory that is under `DrupalFileSystem::getOsTemporaryDirectory()` which will be the case if it is created in `\Drupal\Tests\package_manager\Kernel\PackageManagerKernelTestBase::createTestProject`

and \Drupal\package_manager\PathLocator::getProjectRoot() but we should only delete getProjectRoot() if is directory that is under `DrupalFileSystem::getOsTemporaryDirectory()` which will be the case if it is created in `\Drupal\Tests\package_manager\Kernel

I think we could probably simplify this; if the project root is not the same as $this->getDrupalRoot(), we can delete it. I think this is the case because, in a core clone, $this->getDrupalRoot() is what the path locator will report as the project root, by default, unless we specifically override it.

Need help with the test failure, \Drupal\Tests\package_manager\Kernel\SymlinkValidatorTest::testSymlinkToDirectory is failing with rmdir(/private/tmp/package_manager_testing_roottest95214460/active): Directory not empty, is it because we are trying to delete symlink? or something else?

Right now I am skipping \Drupal\Tests\package_manager\Kernel\StageOwnershipTest::testStageDestroyedWithFileSystemError because we are overriding chmod(), unlink(), rmdir() which we need in tearDown(), is there a way to not skip this test?

We shouldn't use the Drupal file_system service to do the delete, precisely because, in some situations, we override and mock it. Instead, we should use Symfony's Filesystem class, which has a remove() method which can do this for us cleanly.

This looks straightforward but there are some things there that I don't understand why they're necessary...

Answered questions and removed the unlink() calls.

I don't think we want to be modifying permissions of the system-wide temp directory.

If NOT doing that causes some tests to fail, I'd say that points to a flaw in those tests. I will take this on to figure out why we'd need to do that, and fix the affected tests.

I am not able to post comment on GitLab, this is not true I guess, example$staging_root will be /private/tmp/package_manager_testing_roottest30218963/stage so it's parent won't be temp directory.

Assigning to @tedbow for final review.

This looks good. I pushed up 1 commit because there was 1 folder not being deleted. @phenaproxima if this looks good and tests pass, feel free to commit.