Update PHPUnit tests to include messages for assertions

[techanvil]

Feature Description

Following on from #10759, this is an issue to add assertion messages to PHPUnit tests and remove the disabling of the PHPCS rule introduced in the previous issue.

Due to the scope of the changes, we may need to split this into multiple issues.

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• All existing PHPUnit test assertions are updated to include informative failure messages.
• PHPCS rule disabling comments added in Add a PHPCS rule to enforce specifying messages for PHPUnit assertions #10759 to bypass the assertion message rule are removed from test files once all assertions in that file include messages.
• This issue should be treated as a time box which sets a hard limit on the time to spend.
• This will likely take multiple PRs, so follow-up issues should be opened as necessary until completed.
  • If time allows, multiple PRs could be opened against this issue to keep the size of the diff reasonable for review.

Implementation Brief

Test Coverage

QA Brief

Changelog entry

[techanvil]

Thanks @zutigrm this AC looks good, however, noting that there are >3k assertions in over >200 files, I've added the same time-boxing criteria to the AC as seen in #5795.

AC ✅