Fix unstable time based PHPUnit tests

[benbowler]

Bug Description

Despite #6758 and this comment a couple of time based tests were missed and remain in the codebase causing instability:

https://github.com/google/site-kit-wp/blob/4f1d3fb446329ff3022fe3085633763cdd02cc94/tests/phpunit/integration/Core/Prompts/REST_Prompts_ControllerTest.php#L79-L91

https://github.com/google/site-kit-wp/blob/4f1d3fb446329ff3022fe3085633763cdd02cc94/tests/phpunit/integration/Core/Prompts/REST_Prompts_ControllerTest.php#L134-L142

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Acceptance criteria

• Ensure that the tests listed above use the assertEqualsWithDelta assertion for time related keys to ensure better test stability when run on CI.

Implementation Brief

• Split out the use of assertEqualSets in the tests above to separate uses of assertEquals and assertEqualsWithDelta.

Test Coverage

• Ensure tests still cover the original cases and pass.

QA Brief

• QA:Eng to confirm tests cover existing use cases.

Changelog entry

• Fix unstable time based PHPUnit tests.

[benbowler]

I created a candidate PR as this is such a small change.

[eugene-manuilov]

IB ✔️

[zutigrm]

QA:eng Verified

• Did not find other occurrence in the code that uses assertEqualSets with time()
• No tests failed, run it few times