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: http://svn.asterisk.org/svn/asterisk/trunk@392190 f38db490-d61c-443f-a65b-d21fe96a405b