Tests: Add tests for wpdb::_real_escape().
Adds a new dedicated test file.
Adds a test to check that various input types passed to wpdb::_real_escape() are handled correctly.
Note: This new test does not test the actual escaping or other logic in the function. Rather, it just and only tests and documents how the function handles various input types.
Props jrf, hellofromTonya.
See #53363.