tests/test_cel: Add test_cel_attended_transfer_bridges_link to racey tests

Despite failing less often, the ordering of the ATTENDEDTRANSFER event and the
BRIDGE_EXIT event for the Alice and David channels is not defined. This makes
the test still fail.
........

Merged revisions 428918 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@428919 f38db490-d61c-443f-a65b-d21fe96a405b