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)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@398285 65c4cc65-6c06-0410-ace0-fbb531ad65f3