py3: port test-batching.py to python3
I used byteify-strings.py on this file, then manually added the
various pycompat and bprint bits as needed.
Differential Revision: https://phab.mercurial-scm.org/D5678
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed