Remove coverage targets not measured by phpunit.xml.dist

With code coverage on, PHPUnit records every line a test runs. That can make code look tested when it isn't. For example, Url::toString() calls UrlGenerator to build the URL, so a test of Url also runs UrlGenerator's code. Those lines would show as covered, even though the test only checks Url. If UrlGenerator had no tests of its own, the coverage report would still make it look tested, and nobody would know its tests are missing.

To prevent that, a test can specify what code it actually tests with an attribute on the test class: #[CoversClass(Url::class)] for a class, #[CoversTrait] for a trait, #[CoversMethod] for one method and #[CoversFunction] for a function.

That specified code basically becomes the test's coverage target, and PHPUnit only counts lines in the target as covered by that test. UrlGenerator then has to get its coverage from its own tests.

PHPUnit measures the files listed in the <source> section of core/phpunit.xml.dist. Since PHPUnit 12, it also checks that every target is in that list.

For example, WebAssertTest has #[CoversClass(WebAssert::class)], but WebAssert lives in core/tests, which isn't in <source>. When a target is outside the list, PHPUnit throws away the test's coverage and reports a warning. Also, core sets failOnPhpunitWarning="true", and the run fails.

I've been working on making code coverage work and filed two other issues: #3630852: Tests whose coverage target is outside phpunit.xml.dist's source lose their coverage under PHPUnit 12 adds .module and .inc files and the composer/ directory to <source>, which fixes 13 tests. #3630772: PHPUnit 12 deprecated using #[CoversClass] to target traits fixes 25 tests that use #[CoversClass] for a trait, where PHPUnit 12 requires #[CoversTrait]. That leaves about 35 tests with targets that <source> leaves out on purpose:

• 24 cover the test framework in core/tests, like KernelTestBase or WebAssert.
• 8 cover a class in a test module.
• 3 cover a class in core/lib/Drupal/Component, which core's phpunit.xml.dist excludes because the Component tests have their own.

So here is what we have to do:

1. Run all of core's tests with coverage on (takes 8+ hours on my old laptop).
2. Write down every test class with an "is not a valid target for code coverage" warning from PHPUnit.
3. In each of those test classes, delete each Covers attribute that points to a class or method from the warnings. In WebAssertTest, for example, that's #[CoversClass(WebAssert::class)].
4. If a test class has no Covers attribute left, add #[CoversNothing] to it, with use PHPUnit\Framework\Attributes\CoversNothing;, so the code it runs doesn't show up as covered.

(As an aside, Drupal Core already marks four tests #[CoversNothing]. The oldest is TestSiteApplicationTest, from 2018 (#2926633: Provide a script to install a Drupal testsite). The other three are Composer build tests, whose base class got it in 2022 (#3272110: Drupal 9 and 10's Drupal\Component composer.json files are totally out of date). None of those issues discussed adding #[CoversNothing] though.)