Add more new test cases for module inclusion errors

These test cases will also have improved error messages in the next
non-test commit.  They come from the work done to address reviewer
comments for ocaml#10407.

testsuite/tests/typing-gadts/return_type.ml

@@ -0,0 +1,36 @@
+(* TEST
+   * expect
+*)
+
+type i = int
+
+type 'a t = T : i
+[%%expect{|
+type i = int
+Line 3, characters 16-17:
+3 | type 'a t = T : i
+                    ^
+Error: Constraints are not satisfied in this type.
+       Type i should be an instance of 'a t
+|}]
+
+type 'a t = T : i t
+type 'a s = 'a t = T : i t
+[%%expect{|
+type 'a t = T : i t
+Line 2, characters 23-26:
+2 | type 'a s = 'a t = T : i t
+                           ^^^
+Error: Constraints are not satisfied in this type.
+       Type i t should be an instance of 'a s
+|}]
+
+type 'a t = T : i s
+and  'a s = 'a t
+[%%expect{|
+Line 1, characters 16-19:
+1 | type 'a t = T : i s
+                    ^^^
+Error: Constraints are not satisfied in this type.
+       Type i s should be an instance of 'a t
+|}]

testsuite/tests/typing-modules/extension_constructors_errors_test.ml

@@ -42,3 +42,20 @@ Error: Signature mismatch:
          type t += A
        A private type would be revealed.
 |}];;
+
+module M2 : sig type t += A end = struct type t += private A | B end;;
+[%%expect{|
+Line 1, characters 34-68:
+1 | module M2 : sig type t += A end = struct type t += private A | B end;;
+                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Error: Signature mismatch:
+       Modules do not match:
+         sig type t += private A | B  end
+       is not included in
+         sig type t += A end
+       Extension declarations do not match:
+         type t += private A
+       is not included in
+         type t += A
+       A private type would be revealed.
+|}];;