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

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)