unit tests: test_voicemail_api leaks stringfields from snapshots

(closes issue ASTERISK-22414)
Reported by: Corey Farrell
Patches:
    test_voicemail_api-leaks-11.patch uploaded by coreyfarrell (license 5909)
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Merged revisions 398285 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 398286 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398287 65c4cc65-6c06-0410-ace0-fbb531ad65f3