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.
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Merged revisions 428918 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 428919 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@428920 65c4cc65-6c06-0410-ace0-fbb531ad65f3