Add test

Author: liufengyun

tests/pos-special/fatal-warnings/i8781.scala

@@ -0,0 +1,10 @@
+@main
+def Test =
+
+  val x: Int | String = 1
+
+  println(x.isInstanceOf[Int])
+
+  x match
+    case _: Int =>    println("Int")
+    case _: String => println("String")