Tests: Reduce usage of assertEquals for 6.3

[hellofromTonya]

Follow-up to:

• #56800 (6.2)
• #55654 (6.1)
• #54726 (6.0)
• #53364 (5.9)
• #52482 (5.8)
• #38266 (5.7)

The assertEquals() test method does not check that the types of the expected and actual values match. This can hide subtle bugs especially when the values are falsey.

Tasks:

• Switch to using assertSame() when the type of the value in the assertion is important
• Replace overall usage of assertEquals() with type-strict assertion methods, with the aim of potentially removing its usage altogether

To help ease the effort of backporting tests, changes should also be made upstream in the Gutenberg repository.