Add a test suite event to indicate when the atxfer 3-way feature is detected

This adds a test suite event that indicates to tests when the attended transfer
three-way call feature is detected.

Review: https://reviewboard.asterisk.org/r/2912/
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Merged revisions 401578 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@401580 65c4cc65-6c06-0410-ace0-fbb531ad65f3