Add neg tests for clases.

Author: nicolasstucki

tests/neg/phantomInstanceOf2.scala

@@ -2,9 +2,10 @@ import dotty.phantom.PhantomAny
 import dotty.phantom.PhantomNothing
 import dotty.phantom.phantasmic
 
+class Blinky extends PhantomAny
+
 class phantomInstanceOf2 {
-  phantasmic[PhantomAny].asInstanceOf[Any] // error
-  phantasmic[PhantomAny].asInstanceOf[Nothing] // error
-  phantasmic[PhantomNothing].asInstanceOf[Any] // error
-  phantasmic[PhantomNothing].asInstanceOf[Nothing] // error
+  new Blinky().asInstanceOf[Any] // error
+  new Blinky().asInstanceOf[Nothing] // error
+  new Blinky().asInstanceOf[PhantomAny] // error
 }