gtestutils: Bail out of g_test_init() if G_DISABLE_ASSERT is defined

If G_DISABLE_ASSERT is defined, g_assert() is a no-op. Despite it now
being standard practice to *not* use g_assert() in unit tests (use
g_assert_*() instead), a lot of existing unit tests still use it.
Compiling those tests with G_DISABLE_ASSERT would make them silently
no-ops. Avoid that by warning the user loudly.

Note that it’s pretty rare for people to compile with G_DISABLE_ASSERT,
so it’s not expected that this will be hit often.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/976

glib/gtestutils.c

@@ -1274,9 +1274,9 @@ parse_args (gint    *argc_p,
  * Since: 2.16
  */
 void
-g_test_init (int    *argc,
-             char ***argv,
-             ...)
+(g_test_init) (int    *argc,
+               char ***argv,
+               ...)
 {
   static char seedstr[4 + 4 * 8 + 1];
   va_list args;