Bug fix and remove assertion.

get_utc_timestamp() was defining 'minute' as a local loop variable
and hiding the function argument, which was causing the
check_utc_timestamp() test to go screwy in the doomsday scenario.

Also removed the debug assert, since I found a test case to enter here.
Added unit test for imaginary timezone.

Added unit test with an imaginary timezone with a DST offset of
57 minutes, to check the doomsday case in get_utc_timestamp().