Tests: Reduce usage of assertEquals for 6.6

[SergeyBiryukov]

Follow-up to:

• #59655 (6.5)
• #58956 (6.4)
• #57855 (6.3)
• #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 merging tests, changes should also be made upstream in the Gutenberg repository.

This PR replaces assertEquals() with more appropriate, stricter assertions where possible and without making any changes to source.

For easier reviewing, commits have been separated based on the replacement assertion and any additional changes required to implement the stricter assertion.

Trac ticket: https://core.trac.wordpress.org/ticket/60706
Trac ticket: https://core.trac.wordpress.org/ticket/59655
Trac ticket: https://core.trac.wordpress.org/ticket/55654

[SergeyBiryukov]

In 58044:

Tests: Use assertSame() in WP_oEmbed_Controller tests.

This ensures that not only the return values match the expected results, but also that their type is the same.

Going forward, stricter type checking by using assertSame() should generally be preferred to assertEquals() where appropriate, to make the tests more reliable.

Follow-up to [41047].

Props costdev.
See #60706.