Enforce PHPUnit assertion messages

[benbowler]

Summary

Addresses issue:

• Add a PHPCS rule to enforce specifying messages for PHPUnit assertions #10759

Relevant technical choices

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.
• Ensure there are no unexpected significant changes to file sizes.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

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[zutigrm]

Thanks @benbowler LGTM

[eugene-manuilov]

Thanks, @benbowler. Almost all looks good, but there is one thing that we need to address here. Please, take a look.

[eugene-manuilov]

Checking just the Assert class is not enough because we also have a custom assert method defined in the TestCase class that also has the $message argument:

site-kit-wp/tests/phpunit/includes/TestCase.php

Lines 224 to 228 in 44f18cd

	protected function assertArrayIntersection( array $subset, array $array_to_check, $message = '' ) {
	$intersection = array_intersect_key( $array_to_check, $subset );
	$this->assertEqualSetsWithIndex( $subset, $intersection, $message );
	}

We need to make sure such custom assert methods are not ignored.