Add test

Author: odersky

tests/run/extension-override.scala

@@ -0,0 +1,12 @@
+class A:
+  extension (s: String)
+    def len: Int = s.length
+
+object B extends A:
+  extension (s: String)
+    override def len: Int = s.length + 1
+
+@main def Test =
+  import B._
+  assert("abc".len == 4)
+