nixos/tests/xmpp: fix after xmpp >= 1.9

`ClientXMPP.process` is gone but trivial to diy in our case.

see https://slixmpp.readthedocs.io/en/latest/howto/remove_process.html

Willy b80cb24a 8e166f7a

Changed files
+1 -1
nixos
+1 -1
nixos/tests/xmpp/xmpp-sendmessage.nix
···
# MUC
ct.register_plugin('xep_0045')
ct.connect(("${connectTo}", 5222))
-
ct.process(forever=False)
+
ct.loop.run_until_complete(ct.disconnected)
if not ct.test_succeeded:
sys.exit(1)