Code Modernization: Silence the deprecation warning for missing return type in phpunit/tests/compat.php.

This fixes the "Deprecated: Return type of `CountableFake::count()` should be compatible with `Countable::count(): int`" warning on PHP 8.1.

PHP native interfaces now have declared return types and methods in classes implementing these interfaces need to either have the return type declared (in a covariant compatible manner with the PHP native interface method declaration), or need to silence the deprecation warning using the `#[ReturnTypeWillChange]` attribute.

Follow-up to [51517], [51529], [51530], [51531], [51532].

Props jrf.
See #53635.

git-svn-id: https://develop.svn.wordpress.org/trunk@51594 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

tests/phpunit/tests/compat.php

@@ -318,6 +318,7 @@ class ArrayIteratorFake extends ArrayIterator {
 }
 
 class CountableFake implements Countable {
+	#[ReturnTypeWillChange]
 	public function count() {
 		return 16;
 	}