Squiz/StaticThisUsage: improve enum tests

Let's also test that `$this` in a non-static enum method doesn't get flagged.

Author: jrfnl

src/Standards/Squiz/Tests/Scope/StaticThisUsageUnitTest.inc

@@ -117,6 +117,10 @@ $b = new class()
 }
 
 enum MyEnum {
+    private function notStatic () {
+        $this->doSomething();
+    }
+
     public static function myFunc() {
         $this->doSomething();
     }

src/Standards/Squiz/Tests/Scope/StaticThisUsageUnitTest.php

@@ -38,7 +38,7 @@ public function getErrorList()
             80  => 1,
             84  => 1,
             99  => 1,
-            121 => 1,
+            125 => 1,
         ];
 
     }//end getErrorList()