CI issues with gstreamer on macOS

[lnjX]

QDEBUG : tst_QXmppCallManager::testCall() ======== CONNECT ========
QFATAL : tst_QXmppCallManager::testCall() Failed to create rtpbin
FAIL!  : tst_QXmppCallManager::testCall() Received a fatal error.
   Loc: [Unknown file(0)]
Totals: 1 passed, 1 failed, 0 skipped, 0 blacklisted, 878ms
********* Finished testing of tst_QXmppCallManager *********

I don't really know what's going wrong there, maybe some packet is missing? @olesalscheider, maybe you know more? :)

[lnjX]

I decided to skip the test on macOS for now (unfortunately there's another issue now........), see #499.