C++: Change SuppressionScope.toString

This should have no effect in itself but changes the test output to
correspond with the change coming next.

cpp/ql/src/AlertSuppression.ql

@@ -77,7 +77,7 @@ class SuppressionScope extends @comment {
 
   /** Gets a textual representation of this element. */
   string toString() {
-    result = "suppression range"
+    result = this.getSuppressionComment().toString()
   }
 }