res_parking: Unit tests

Adds the following unit tests:
* create_lot: tests adding and removal of a new parking lot (baseline)
* park_extensions: creates a parking lot that registers extensions and
      then confirms that all of the expected extensions exist
* extensions_conflicts: creates numerous parking lots to test that
      extension conflicts in parking lots result in parking lot
      creation failing
* dynamic_parking_variables: Tests that the creation of dynamic
      parking lots respects the related channel variables set on the
      channel that requests them.
* park_call: Tests adding a channel to a parking lot's holding bridge
      by standard parking functions.
* retrieve_call: Tests pulling a channel out of a parking lot's
      holding bridge via parked call retrieval functions.

(closes issue ASTERISK-22138)
Reported by: Matt Jordan
Review: https://reviewboard.asterisk.org/r/2714/



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396175 65c4cc65-6c06-0410-ace0-fbb531ad65f3