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.