Tests: Improves the documentation and structure of test cases

[mukeshpanchal27]

Trac ticket: https://core.trac.wordpress.org/ticket/26798

This PR improves the documentation and structure of test cases related to post date handling in tests/phpunit/tests/post.php. The main focus is on updating docblocks for clarity and converting data provider methods to static methods, which is a best practice in PHPUnit.

• Updated the docblocks for test_wp_insert_post_handle_malformed_post_date() and test_wp_resolve_post_date_regex() to clearly describe the parameters and expected behavior of each test, making the intent and usage of the tests more understandable.
• Converted the data provider methods data_wp_insert_post_handle_malformed_post_date() and data_wp_resolve_post_date_regex() to static methods and enhanced their docblocks to specify the structure of the returned data, aligning with PHPUnit best practices.

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[westonruter]

Ideally this would be:

Suggested change

	* @return array<array{ date: string, expected: bool }>
	* @return array<string, array{ date: string, expected: bool }>

Where the array keys in the data provider below are strings in an associative array. What are currently comments like "24-hour time format" should rather be part of the array key. This would make it much easier to locate the data set for a failing test.

[westonruter]

Ditto above:

Suggested change

	* @return array<array{ date: string, expected: string|false }>
	* @return array<string, array{ date: string, expected: string|false }>

But this would require coming up with names for all of the tests to use in the keys.