Follow-up for #3039568: ReadOnlyModeUpdateTest's namespace is defined incorrectly, causing the test not to run

#3039568-60: Add a read-only mode to JSON:API added an update path test: ReadOnlyModeUpdateTest. It passes tests fine locally.

JSON:API's test suite passes just fine, both on #3039568: Add a read-only mode to JSON:API and on https://www.drupal.org/node/2723491/qa.

But when ported to core, it fails: #2843147-171: Add JSON:API to core as a stable module.

Queue surprise.

I re-tested the core patch. Consistently failing. I re-tested locally. Consistently passing. In different PHP installations.

Queue despair.

Locally, I was testing with the PHPUnit test runner. DrupalCI is testing with run-tests.sh. It took me 45 minutes of debugging run-tests.sh to figure out the correct incantation to run a single class. 😡 php core/scripts/run-tests.sh --url http://d8.test --module jsonapi --suppress-deprecations --non-html --concurrency "12" has worked fine for months, but replacing --module jsonapi with --group jsonapifoobar doesn't work, nor does --file path/to/file, nor does --class F/Q/C/N. Turns out that for everything except --module, it needs to be at the end of the arguments … (ﾉಥ益ಥ）ﾉ ┻━┻ 🤯

But even php core/scripts/run-tests.sh --url http://d8.test --suppress-deprecations --non-html --concurrency "12" --file core/modules/jsonapi/tests/src/Functional/Update/ReadOnlyModeUpdateTest.php didn't work. I had to debug run-tests.sh further, to figure out why I was getting ERROR: No valid tests were specified.. At some point, it checks if is_subclass_of($namespace_class, TestCase::class). That turned out to be FALSE too. Which led to me inspecting the class hierarchy, comparing with other core test that now did work appropriately, and then I saw it.

-namespace Drupal\jsonapi\Tests\Functional\Update;
+namespace Drupal\Tests\jsonapi\Functional\Update;

And yes, inspecting the test runs for #3039568 and the project-level test runs, I can see that the new test simply isn't being run. This is very easy to miss though.

It's still a mystery to me why only the core test runner detects this problem. I wonder how many contrib modules have tests that do work fine locally, in their maintainers' IDE, yet don't actually run on DrupalCI at all…