fix(tests): Attempt to fix unstable `test_unmerge` test

This test is failing because the merge hasn't finished in snuba by the time we make the query. We
can tell this because it receives `set([('blue', 2), ('green', 2), ('red', 2)])`, which are the tags
that were originally created on this group.

Testing just dropping the date down a bunch. I'm not sure this will help, the other option might be
a brief sleep before we query the tags, or potentially retry a few times?

tests/snuba/tasks/test_unmerge.py

@@ -169,7 +169,7 @@ class UnmergeTestCase(TestCase, SnubaTestCase):
         }
 
     def test_unmerge(self):
-        now = before_now(seconds=20).replace(microsecond=0, tzinfo=pytz.utc)
+        now = before_now(minutes=5).replace(microsecond=0, tzinfo=pytz.utc)
 
         def time_from_now(offset=0):
             return now + timedelta(seconds=offset)