Test case for stale overloading

Show that toplevel definitions on several files do not
combine as overloaded denotations. Instead, the youngest
definition wins.

Author: odersky

tests/run/toplevel-stale/A_1.scala

@@ -0,0 +1 @@
+def foo(x: Int): String = "old"

tests/run/toplevel-stale/B_2.scala

@@ -0,0 +1 @@
+def foo(x: Long): String = "new"

tests/run/toplevel-stale/Test_3.scala

@@ -0,0 +1,2 @@
+@main def Test() =
+  assert(foo(1) == "new")