Specify UTC time zone for the test suite (#9945)

Specify UTC time zone for the test suite (#9945)

The controller.bar/not-grouped/on-time test was failing on my computer because the date ranges happen to cross the end of Daylight Saving Time in the U.S., so chart was generated with one more hour of time than the test fixture expected.

Using moment-timezone to specify a fixed time zone with no DST seemed like the most robust fix. (Alternatively, I could pick a date range that doesn't change DST; that ought to work.)