Using testing.services.yml in a kernelTest fatals when trying to copy this to the test site

Yes, this makes sense. For context, this is how $this->siteDirectory is set up:

    $this->siteDirectory = vfsStream::url('root/' . $test_site_path);

so it ends up being something like vfs://root/sites/simpletest/123456789. Since $this->root is the actual (host) Drupal root URL the prepending doesn't make any sense.

Regarding test coverage: Yeah, I don't think we can get away with modifying anything in sites/default as part of a test. If people actually use that feature and happen to run KernelTestBaseTest we would either be deleting their test overrides or the test would fail due to permission issues. Neither is desirable, IMO. So let's just not test this. The fact that this explicitly involves the host/parent system is also the reason why this is currently completely untested. So far no one has been able to think of a way to test this in isolation...

Also, I found #2927487: \Drupal\KernelTests\KernelTestBase::bootEnvironment() has some dead code while reviewing this.

Thanks, perfect!

To make up for the lack of automated test coverage, here's the output my shell where I verified the correctness of the patch:

/path/to/drupal/core/tests $ php ../../vendor/bin/phpunit --configuration ../ 'Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest'
PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

Testing Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest
.

Time: 2.6 minutes, Memory: 6.00MB

OK (1 test, 552 assertions)
/path/to/drupal/core/tests $ touch ../../sites/default/testing.services.yml
/path/to/drupal/core/tests $ php ../../vendor/bin/phpunit --configuration ../ 'Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest'
PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

Testing Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest
E

Time: 958 ms, Memory: 6.00MB

There was 1 error:

1) Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest::testCoreLibraryCompleteness
copy(/path/to/drupal/vfs://root/sites/simpletest/41735229/services.yml): failed to open stream: No such file or directory

/Users/tobias.stoeckler/workspace/d8/vendor/symfony/phpunit-bridge/DeprecationErrorHandler.php:262
/Users/tobias.stoeckler/workspace/d8/core/tests/Drupal/KernelTests/KernelTestBase.php:331
/Users/tobias.stoeckler/workspace/d8/core/tests/Drupal/KernelTests/KernelTestBase.php:243
/Users/tobias.stoeckler/workspace/d8/core/tests/Drupal/KernelTests/Core/Asset/ResolvedLibraryDefinitionsFilesMatchTest.php:95

FAILURES!
Tests: 1, Assertions: 0, Errors: 1.
/path/to/drupal/core/tests $ curl https://www.drupal.org/files/issues/2927476-4.patch | git apply --index
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1034  100  1034    0     0   8970      0 --:--:-- --:--:-- --:--:--  8991
/path/to/drupal/core/tests $ php ../../vendor/bin/phpunit --configuration ../ 'Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest'
PHPUnit 4.8.36 by Sebastian Bergmann and contributors.

Testing Drupal\KernelTests\Core\Asset\ResolvedLibraryDefinitionsFilesMatchTest
.

Time: 2.64 minutes, Memory: 6.00MB

OK (1 test, 552 assertions)

BTW, also found #2927487: \Drupal\KernelTests\KernelTestBase::bootEnvironment() has some dead code when testing this locally.

Thanks for the test patch and the verification in #6; those demonstrate the bug and fix nicely. I agree we should not futz with sites/default in a test. (Reminds my of my first core issue... #1212992: Prevent tests from deleting main installation's tables when parent::setUp() is not called.)

The silly concatenation fix is out of scope and it did actually distract me from the fix, because I spent a couple minutes trying to figure out why that also needed a change before I saw it was just a silly out-of-scope cleanup. But I will let it slide just this once. :P

Committed to 8.5.x, and cherry-picked to 8.4.x as a test-only bugfix. Thanks!