Fix the test_substitution test

In r391947, the CDR function was modified such that it will return a
value for the start,answer, and end times if asked. That time will just
be 0 if it hasn't happened yet.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@392190 65c4cc65-6c06-0410-ace0-fbb531ad65f3