tests: Ignore -Wdiscarded-qualifiers with volatile atomics tests

GCC 14 now emits this warning with the tests:
```
In file included from ../glib/gthread.h:34,
                 from ../glib/gasyncqueue.h:34,
                 from ../glib/glib.h:34,
                 from ../glib/tests/atomic.c:14:
../glib/tests/atomic.c: In function 'test_types':
../glib/gatomic.h:140:5: error: argument 2 of '__atomic_store' discards 'volatile' qualifier [-Werror=discarded-qualifiers]
  140 |     __atomic_store (gaps_temp_atomic, &gaps_temp_newval, __ATOMIC_SEQ_CST); \
      |     ^~~~~~~~~~~~~~
../glib/tests/atomic.c:139:3: note: in expansion of macro 'g_atomic_pointer_set'
  139 |   g_atomic_pointer_set (&vp_str_vol, NULL);
      |   ^~~~~~~~~~~~~~~~~~~~
cc1.exe: all warnings being treated as errors
```

I can’t think of a way to cast around this in the definition of
`g_atomic_pointer_set()` without making the behaviour worse (less type
safe) for modern non-volatile atomic variables.

We would like to strongly nudge users of GLib away from declaring atomic
variables as `volatile`, so letting another compiler warning be emitted
when they do is not the end of the world. As long as it doesn’t stop old
code compiling (without `-Werror`).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/tests/atomic.c

@@ -136,6 +136,7 @@ test_types (void)
    * to make sure that we don’t warn when built against older third party code. */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
+#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
   g_atomic_pointer_set (&vp_str_vol, NULL);
   g_atomic_pointer_set (&vp_str, str);
   res = g_atomic_pointer_compare_and_exchange (&vp_str_vol, NULL, str);