suppress compiler warnings in _ctypes_test (#902)

Changed test code to suppress a compiler warning, while taking care to avoid the code being optimized out by the compiler.

Modules/_ctypes/_ctypes_test.c

@@ -52,9 +52,9 @@ _testfunc_cbk_large_struct(Test in, void (*func)(Test))
 EXPORT(void)
 _testfunc_large_struct_update_value(Test in)
 {
-    in.first = 0x0badf00d;
-    in.second = 0x0badf00d;
-    in.third = 0x0badf00d;
+    ((volatile Test *)&in)->first = 0x0badf00d;
+    ((volatile Test *)&in)->second = 0x0badf00d;
+    ((volatile Test *)&in)->third = 0x0badf00d;
 }
 
 EXPORT(void)testfunc_array(int values[4])