tests: remove unused IgnoreTest exception
AFAICT its last use was removed in
d839e4820da7.
tests: use unittest.SkipTest
unittest.SkipTest was introduced in Python 2.7. We previously defined
it with our own class so we could run on Python 2.6.
changegroup: let callers pass in transaction to apply() (API)
I think passing in the transaction makes it a little clearer and more
consistent with bundle2.