test_substitution: Fix failing test.

Revert the -r392190 change.  The original test was correct.  The CDR code
was actually returning an unititialized buffer.
........

Merged revisions 398025 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398026 65c4cc65-6c06-0410-ace0-fbb531ad65f3