Fix edge case when modifying thread tags; don't mark system test as xfail.

When modifying thread tags, we previously had some logic that tried to not
match against tag public ids if the input wasn't a valid public ID. But it
wasn't quite right, and in any case tag public IDs are special snowflakes in
that they can be any string! So just remove this. Also enable the system test
for modifying custom tags, which should after all work now.