flambda-backend: Add tests for #2162 (#2170)

Author: Ekdohibs

testsuite/tests/typing-local/local_optparam.ml

@@ -0,0 +1,6 @@
+(* TEST *)
+
+let foo (local_ f) l = l
+let go a b g ?(x = foo (fun y -> a, y) b) () =
+  let x = foo (fun y -> a, y) x in
+  g x