add test for module type decl in signature item

<!-- ps-id: d0989bfd-2d2a-4182-89bb-40edf8006617 -->

Author: tatchi

src_test/ppx_deriving/test_intf.ml

@@ -1 +1,5 @@
 [%%import: type a = Stuff.a]
+
+module type Example = sig
+  module type InnerModule = [%import: (module Stuff.S_optional)]
+end

src_test/ppx_deriving/test_intf.mli

@@ -1 +1,5 @@
 [%%import: type a = Stuff.a]
+
+module type Example = sig
+  module type InnerModule = [%import: (module Stuff.S_optional)]
+end