Add some tests for invalid subtyping due to non-matching field types (WebAssembly#573)

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.

Author: fitzgen

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 (ref null none)))))
+    (type $b (sub $a (struct (field (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 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      (ref null any)))))
+    (type $d (sub $c (struct (field (mut (ref null any))))))
+  )
+  "sub type 1 does not match super type"
+)