tests: fix module-import warnings in test-update-atomic.t
These were caught by test-check-module-imports.t on Python 3.
This should also make python3-buildbot green again.
Differential Revision: https://phab.mercurial-scm.org/D5674
py3: use dict.items() instead of dict.iteritems() in tests/test-lfs-server.t
dict.iteritems() is not present on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D5673