tests: Add a GAsyncReadyCallback that stores the GAsyncResult

It seems I eventually introduce this into every project where I've
added GLib-based unit tests. Today it's dbus' turn.
Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=105656

test/test-utils-glib.c

@@ -819,3 +819,19 @@ test_check_tcp_works (void)
   return TRUE;
 #endif
 }
+
+/*
+ * Store the result of an async operation. @user_data is a pointer to a
+ * variable that can store @result, initialized to %NULL.
+ */
+void
+test_store_result_cb (GObject *source_object G_GNUC_UNUSED,
+                      GAsyncResult *result,
+                      gpointer user_data)
+{
+  GAsyncResult **result_p = user_data;
+
+  g_assert_nonnull (result_p);
+  g_assert_null (*result_p);
+  *result_p = g_object_ref (result);
+}

test/test-utils-glib.h

@@ -123,4 +123,8 @@ backported_g_steal_pointer (gpointer pointer_to_pointer)
 
 gboolean test_check_tcp_works (void);
 
+void test_store_result_cb (GObject *source_object,
+                           GAsyncResult *result,
+                           gpointer user_data);
+
 #endif