Add the TESTTIME() dialplan function, which permits testing GotoIfTime.

Specifically, by setting TESTTIME() to a particular date and time, you
can test whether a dialplan correctly branches as was intended.  This was
developed after recent questions on the -users list on how to test their
holiday dialplan logic.
(closes issue #16464)
 Reported by: tilghman
 Patches: 
       20100112__issue16464.diff.txt uploaded by tilghman (license 14)
 
Review: https://reviewboard.asterisk.org/r/458/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@239957 65c4cc65-6c06-0410-ace0-fbb531ad65f3