Deprecate support for `#[CoversTrait]` and `#[UsesTrait]` attributes

[sebastianbergmann]

The #[CoversTrait] and #[UsesTrait] attributes were introduced in PHPUnit 11.2 while/after working on #5798.

Had I been aware at the time that #[CoversClass] and #[UsesClass] also target the traits used by the targeted classes, I would not have implemented the #[CoversTrait] and #[UsesTrait] attributes. As of 17bbefa, this behaviour is now documented in tests.

The #[CoversTrait] and #[UsesTrait] attributes are therefore not needed and will be deprecated in PHPUnit 11.4 and removed in PHPUnit 12.

If you currently use #[CoversClass] or #[UsesClass] to target traits: don't. Testing traits in isolation from their embedding classes is not useful.

If you currently use #[CoversTrait] or #[UsesTrait] to target traits: don't. The traits used by the class(es) you target with #[CoversClass] or #[UsesClass] will be targeted as well.

[dshafik]

@sebastianbergmann I'd like to re-visit this change. It may be my miss-understanding, but I think there is value in this feature as it previously worked.

Specifically, I want to limit coverage counting to the code in a specific trait, to ensure I'm not accidentally marking other code in the class as covered when executing it without any assertions on its behavior (e.g. during the arrange part of the test).

It is a common pattern to group a feature into a single trait, and then have a test file for just that feature.

I believe the use of #[CoversTrait] is superior to #[CoversClass] in this instance and leads to more accurate code coverage numbers and no accidental coverage.

[sebastianbergmann]

I reverted the deprecation in 11.5 and the removal in main.