Helpers\IsUnitTestTrait: bug fix - allow for custom test classes passed as FQN

[jrfnl]

While looking at an old PR (#1960), I realized that custom test classes passed as FQN were not handled correctly by the trait. This code was not adjusted in that PR, just moved, so this bug has existed for a while.

The problem was in this code snippet:

		$known_test_classes = RulesetPropertyHelper::merge_custom_array(
			$this->custom_test_classes,
			$this->known_test_classes
		);

		/*
		 * Show some tolerance for user input.
		 * The custom test class names should be passed as FQN without a prefixing `\`.
		 */
		foreach ( $known_test_classes as $k => $v ) {
			$known_test_classes[ $k ] = ltrim( $v, '\\' );
		}

After the merge of the custom classes, the $known_test_classes array is in string => bool format, so the foreach() loop is absolutely not having the intended effect. The only thing it does, is change the boolean value to strings...

I also noticed that the array merge was being done every single time the is_test_class() method was being called. We can make that a bit more efficient as it will be rare that the list of custom tests classes changes during a run.

So, I have made the following changes:

• Introduce a new get_all_test_classes() method which will handle the cleaning of the custom names and the merge. This new class uses two new properties $added_custom_test_classes and $all_test_classes to prevent having to do the same merge over and over again.
• The $known_test_classes property has now been made private.
• The logic to trim namespace separators of custom test class names has been moved to before the merge as we don't need to trim the names from the $known_test_classes.
• The logic to trim namespace separators will now work correctly as the value of the $custom_test_classes will be string[] and hasn't been "flipped" yet (keys/values reversed).

Includes additional unit tests.

[GaryJones]

✅