Fix #686: Move testcase to neg/

The corresponding feature in scalac only ever worked under
-Xexperimental and was dropped in scala/scala#3938

Author: smarter

tests/pending/run/t8764.scala renamed to tests/neg/t8764.scala

@@ -3,14 +3,14 @@ case class IntOnly(i: Int, j: Int)
 
 println("IntOnly: should return an unboxed int")
 val a = IntOnly(1, 2)
-val i: Int = a.productElement(0)
+val i: Int = a.productElement(0) // error: found Any, required Int
 println(s"Int: ${a.productElement(0).getClass}")
 
 case class IntAndDouble(i: Int, d: Double)
 
 println("IntAndDouble: should just box and return Anyval")
 val b = IntAndDouble(1, 2.0)
-val j: AnyVal = b.productElement(0)
+val j: AnyVal = b.productElement(0) // error: found Any, required AnyVal
 println(s"Double: ${b.productElement(1).getClass}")
 println(s"Int: ${b.productElement(0).getClass}")
 }