tests: Undef G_DISABLE_ASSERT
Replacing g_assert with g_assert_true is no sufficient, since g_test_init will abort if G_DISABLE_ASSERT is defined anyway, so let's just undef it, as advised in g_test_init documentation.
Especially required since xfce4-dev-tools!108 (merged).
Fixes: #110 (closed)
Edited by Gaël Bonithon