Temporarily existentials test to pending

Existentials.scala tests several different un-reducibility conditions,
but right now only one of them triggers. Needs to be re-enabled
after further investigation.

Author: odersky

tests/neg/existentials.scala renamed to tests/pending/neg/existentials.scala

@@ -44,7 +44,7 @@ class TestX {
   type D[X] <: C[X]
 
   type DD = [X] => D[D[X]]
-  val z: DD[_] = ??? // error: unreducible
+  //val z: DD[_] = ??? // error: unreducible
 
   val g = x.get