Djblets has some utility functions for working with dates/timestamps

that did not have any unit tests for them.

The cursor of some text fields in an edit state displayed the standard

Added new DatesTests class to djblets/util/tests.py that covers

testing for:
Calling http_date():

- With a datetime object.

- With a string representing a date.

- With an integer representing a Unix timestamp (seconds since the 

epoch).
Calling get_latest_timestamp():

- Without any timestamps in the list (result should be None)

- With a list of timestamps of different values in a jumbled, 

unsorted order, checking that the latest gets returned.
Calling get_tz_aware_utcnow() and checking that the resulting 

datetime's timezone is UTC

Added tests