Code review comment for lp://staging/~jml/subunit/filter-tags

I merged this to a worktree tree of trunk. There was a trivial conflict (mis-bound hunks in test_results.py), but tests failed:

======================================================================
FAIL: test_time_ordering_preserved (subunit.tests.test_subunit_filter.TestTestResultFilter)
subunit.tests.test_subunit_filter.TestTestResultFilter.test_time_ordering_preserved
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/home/robertc/source/unittest/subunit/working/python/subunit/tests/test_subunit_filter.py", line 240, in test_time_ordering_preserved
    ('time', date_c)], result._events)
AssertionError: Sequences differ: [('time', datetime.datetime(20... != [('time', datetime.datetime(20...

First differing element 1:
('time', datetime.datetime(2000, 1, 2, 0, 0, tzinfo=<subunit.iso8601.Utc object at 0x194af10>))
('startTest', <subunit.RemotedTestCase description='foo'>)

+ [('time', datetime.datetime(2000, 1, 1, 0, 0, tzinfo=<FixedOffset '+00:00'>)),
- [('time',
- datetime.datetime(2000, 1, 1, 0, 0, tzinfo=<subunit.iso8601.Utc object at 0x194af10>)),
- ('time',
- datetime.datetime(2000, 1, 2, 0, 0, tzinfo=<subunit.iso8601.Utc object at 0x194af10>)),
   ('startTest', <subunit.RemotedTestCase description='foo'>),
+ ('time', datetime.datetime(2000, 1, 2, 0, 0, tzinfo=<FixedOffset '+00:00'>)),
   ('addError', <subunit.RemotedTestCase description='foo'>, {}),
   ('stopTest', <subunit.RemotedTestCase description='foo'>),
+ ('time', datetime.datetime(2000, 1, 3, 0, 0, tzinfo=<FixedOffset '+00:00'>))]
- ('time',
- datetime.datetime(2000, 1, 3, 0, 0, tzinfo=<subunit.iso8601.Utc object at 0x194af10>))]