gajim: Enable running test suite

This is a bit more involved, because first of all, the tests aren't
distributed in the release tarball and second the test suite currently
doesn't work but there are fixes for it that get released in the next
upstream patch, so we cherry-pick the relevant commits as patches.

We now also switch to fetching the tarball directly from their GitLab
instance, because - as mentioned - it contains the tests and also
contains the icon.index file, which we already had included as a patch
and we can now drop it.

The URLs to the cherry-picked upstream commits are the following:

https://dev.gajim.org/gajim/gajim/commit/1f0d7387fd020df5dfc9a6349005ec7dedb7c008
https://dev.gajim.org/gajim/gajim/commit/491d32a2ec13ed3a482e151e0b403eda7b4151b8
https://dev.gajim.org/gajim/gajim/commit/46a19733d208fbd2404cbaeedd8c203d0b6557a4



All of these commits are in the gajim_0.16 branch and are thus very
likely becoming part of Gajim 0.16.7.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @7c6f434c