Add some tests for invalid subtyping due to non-matching field types

Specifically around the requirements that both field types have the same
mutability and that the storage type must be exactly the same when the field
type is mutable.

test/core/gc/type-subtyping-invalid.wast

@@ -0,0 +1,37 @@
+(assert_invalid
+  (module
+    ;; When fields are mutable, a subtype's reference fields cannot be subtypes of
+    ;; the supertype's fields, they must match exactly.
+    (type $a (sub    (struct (field (mut (ref null any))))))
+    (type $b (sub $a (struct (field (mut (ref null none))))))
+  )
+  "sub type 1 does not match super type"
+)
+
+(assert_invalid
+  (module
+    ;; When fields are const, a subtype's reference fields cannot be supertypes of
+    ;; the supertype's fields, they must be subtypes.
+    (type $a (sub    (struct (field (mut (ref null none))))))
+    (type $b (sub $a (struct (field (mut (ref null any))))))
+  )
+  "sub type 1 does not match super type"
+ )
+
+(assert_invalid
+ (module
+   ;; The mutability of fields must be the same.
+   (type $c (sub    (struct (field (mut (ref null any))))))
+   (type $d (sub $c (struct (field      (ref null none)))))
+  )
+  "sub type 1 does not match super type"
+)
+
+(assert_invalid
+ (module
+   ;; The mutability of fields must be the same.
+   (type $c (sub    (struct (field      (ref null any)))))
+   (type $d (sub $c (struct (field (mut (ref null none))))))
+  )
+  "sub type 1 does not match super type"
+)