Allow the same test-specific overrides in KernelTestBase as in WebTestBase

Here we go. This still needs (automated) tests, but I verified that this works locally.

So this was reported by @chx, who needs this to be able to properly test the MongoDB suite. As mentioned in #2229011: Tests are no longer modifiable there are other possible use-cases (those apply to KernelTestBase tests as much as WebTestBase).

Also note that it's totally my fault that this currently does not work. I implemented the testing.services.yml support for KernelTestBase tests in the referenced issue, but I cannot fathom why I stopped at the halfway mark.

Don't want to keep anyone from working on this, but keeping this on my radar.

Thanks! This being assigned to me is a lie at this point... ..sadly. Hope I manage to get back to this soon.

The test looks great. It should generally be sufficient in terms of coverage IMO although SimpleTestTest contains

# Make sure that the $test_class variable is defined when this file is included.
if ($test_class) {
}

in the test settings.testing.php. I think that could easily be added here without any further assertions.

I have one minor nitpick:

+    file_put_contents($settings_testing_file, $settings_file);

The variable names are a bit confusing, because the first is a filepath and the second are the file contents but the names are very similar.

Awesome, thanks!