Test tool and unit test improvements for 6.8

[desrosj]

• #61530 (6.7)
• #60705 (6.6)
• #59647 (6.5)
• #58955 (6.4)
• #57841 (6.3)
• #56793 (6.2)
• #55652 (6.1)
• #54725 (6.0)
• #53363 (5.9)
• #52625 (5.8)
• #51802 (5.7)
• #51344 (5.6)

This ticket is for various fixes and improvements in PHPUnit tests that don't have a more specific ticket, as well as general improvements to the GitHub Actions workflows that run automated testing.

[desrosj]

In 59282:

Build/Test Tools: Test against MySQL 9.0.

Version 9.0 is the latest short-term innovation release of MySQL.

See #62280.

[SergeyBiryukov]

In 59408:

Tests: Add missing @covers tag for fetch_feed() tests.

Includes correcting the test class name as per the naming conventions.

Follow-up to [59382].

See #62280.