Squiz/ClassFileName: improve enum tests

Add a _valid_ test case for a backed enum.

Note: this new test will fail until PR 3534 has been merged.

Author: jrfnl

src/Standards/Squiz/Tests/Classes/ClassFileNameUnitTest.inc

@@ -6,7 +6,7 @@ class ClassFileNameUnitTest {}
 interface ClassFileNameUnitTest {}
 trait ClassFileNameUnitTest {}
 enum ClassFileNameUnitTest {}
-
+enum ClassFileNameUnitTest: int {}
 
 // Invalid filename matching class name (case sensitive).
 class classFileNameUnitTest {}