test(tests/lean/run): add overload test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

tests/lean/run/over2.lean

@@ -0,0 +1,14 @@
+import data.nat
+using nat
+
+namespace N1
+  definition foo (a : nat) := a
+end N1
+
+namespace N2
+  definition foo (a : nat) := a
+end N2
+
+using N1 N2
+
+definition boo := foo