gajim: Re-enable tests

Just because the tests need GTK+, I think it's not a good idea to simply
disable them when we're still able to use an X virtual framebuffer to
run them anyway.

As with the package we had prior to version 1.0 I've disabled the
resolver test, because this one requires networking.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @abbradar, @Mic92, @7c6f434c