Added automated test for UTC date ticks and labels.

The date tick tests are broken in the automatic ticker test suite
because they assume EST timezone. In addition, the suite is disabled.
Add a simple automatic test for the new option in a separate test:

  * `auto-tests/tests/date_ticks.js`:
    Simple test for the generation of ticks and labels at nice time
    positions according to UTC as set by option `labelsDateUTC`.