YoastCS rules: enforce final classes for all test files

... while allowing `abstract` TestCases.

Note: the typical/default "Doubles" directories within the "tests" directory are exempt from this rule as "Double" classes are often still combined with mocking and making the class `final` would break those tests.

Related to 303

Impact on Yoast packages:

| Plugin/Tool       | Errors/Warnings |
|-------------------|-----------------|
| PHPUnit Polyfills | 25 (fixtures for the tests, these will be exempt)
| WP Test Utils     | 9 (mostly fixtures for the tests)
| YoastCS           | --
| WHIP              | 9
| Yoast Test Helper | --
| Duplicate Post    | 25
| Yst ACF           | 10
| Yst WooCommerce   | 25
| Yst News          | 13
| Yst Local         | 11
| Yst Video         | 101
| Yst Premium       | 126
| Yst Free          | 647

.phpcs.xml.dist

@@ -68,7 +68,10 @@
 	</rule>
 
 	<!-- Enforce Final classes to prevent issues with the PHPCS autoloader. -->
-	<rule ref="Universal.Classes.RequireFinalClass"/>
+	<rule ref="Universal.Classes.RequireFinalClass">
+		<!-- YoastCS only applies this rule to test files. Overrule it to apply to all files. -->
+		<include-pattern>*\.php$</include-pattern>
+	</rule>
 
 
 	<!--

Yoast/ruleset.xml

@@ -483,4 +483,11 @@
 		<exclude name="PSR1.Classes.ClassDeclaration.MultipleClasses"/>
 	</rule>
 
+	<!-- Enforce that all test classes are either final or abstract. -->
+	<rule ref="Universal.Classes.RequireFinalClass">
+		<include-pattern>*/tests/*\.php$</include-pattern>
+		<!-- But don't enforce this for Mock/Double classes. -->
+		<exclude-pattern>*/tests(/*)?/Doubles/*\.php$</exclude-pattern>
+	</rule>
+
 </ruleset>