Add missing negative test.

Author: odersky

tests/neg/abstract-override.scala

@@ -0,0 +1,8 @@
+trait T { def foo: Int }
+trait T1 extends T { override def foo = super.foo }
+trait T2 extends T { override def foo = super.foo }
+object Test extends T2 with T1 {
+  def main(args: Array[String]) = {
+    assert(foo == 3)
+  }
+}