Fix CI: Make test fail

Author: liufengyun

tests/neg-custom-args/fatal-warnings/i12188/Test.scala

@@ -3,3 +3,7 @@ case class PC1(a: String) extends P
 case class PC2(b: Int) extends P
 
 def Test = MatchTest.test(PC2(10): P)
+
+def foo(x: P): Unit =
+  x match           // error
+  case _: PC1 =>