Add Tests for Search highlight & Fix issue where EP returns incorrect excerpt

[burhandodhy]

Description of the Change

This PR fixes the issue where EP returns the incorrect excerpt when the Excerpt highlighting option is enabled and get_the_excerpt is called without the Post Loop.

Closes #3101

How to test the Change

1. Enable Excerpt highlighting option
2. Create a post and leave the excerpt empty
3. Search for that post through the code.

        $posts = get_posts( [ 's' => 'searchTerm' ] );
	var_dump( get_the_excerpt( $post[0] ) );

Changelog Entry

Fixed - Incorrect Excerpt when get_the_excerpt is called without Loop and Excerpt highlighting option is enabled

Credits

Props @burhandodhy

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.