PHPUnit tests: Add/improve @covers annotations for WP 6.6

[pbearne]

Previously:

• #58851 (6.4)
• #57958 (6.3)
• #56782 (6.2)
• #39265 (up to 6.1)

Continuation of ongoing work to add and/or improve the PHPUnit tests @covers annotations for more accurate coverage reporting.

[swissspidy]

There are tons of uncommitted PRs from the previous ticket, #58851.

Would be good to get these all reviewed & committed one by one.

[hellofromTonya]

My plan: finish the current scope of work (ie the current patch) in March.

But with 6.5 RC1 happening in the next hour or so, moving this to 6.6 and repurposing the ticket to 6.6 cycle.

[hellofromTonya]

There's a big change coming for @covers for the test suite to be compatible with the newest PHPUnit versions. More details will be coming in a follow-up ticket. The tl;dr version: @covers will not be declared at the test method and will be replaced with a test class level attribute (​see PHPUnit Code Coverage).

This ticket and its PRs will be closed once a proposal and ticket(s) are opened for the new scope of work.

For now, moving this ticket to Future Release, though I'd suggest no further work or commits happen for it.