Fix PHPUnit problems regarding @covers and @use

[jipmoors]

While trying to get an insight in the Tests that are implemented in WordPress, we came across some malformed @covers and @use statements.

This patch fixes these small problems.

Functions in the global namespace should be prefixed with :: for PHPUnit to understand they are global. This is not very well documented, but has been used in the codebase already on other tests.

[SergeyBiryukov]

In 42636:

Tests: Correct erroneous @covers and @use tags.

Props jipmoors.
Fixes #43203.