Add proptests for structs with mixed members (copy/noncopy)

tests/structs.witx

@@ -11,6 +11,11 @@
     (field $first (@witx const_pointer s32))
     (field $second (@witx const_pointer s32))))
 
+(typename $pair_int_and_ptr
+  (struct
+    (field $first (@witx const_pointer s32))
+    (field $second s32)))
+
 (module $structs
   (@interface func (export "sum_of_pair")
     (param $an_pair $pair_ints)
@@ -20,4 +25,8 @@
     (param $an_pair $pair_int_ptrs)
     (result $error $errno)
     (result $doubled s64))
+  (@interface func (export "sum_of_int_and_ptr")
+    (param $an_pair $pair_int_and_ptr)
+    (result $error $errno)
+    (result $double s64))
 )