Build/Test Tools: Use strict assertions in various test cases

[Soean]

What

Replaces loose assertions (assertEquals() / assertNotEquals()) with their strict counterparts (assertSame() / assertNotSame()) across a number of PHPUnit test files.

Why

Strict assertions additionally check the type of the compared values, making the tests more robust against unintended type coercion and better documenting the expected return types of the functions under test.

Testing

All affected test files pass locally: OK (71 tests, 669 assertions).

Trac ticket: https://core.trac.wordpress.org/ticket/64895

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[github-actions]

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