tests: make test-manifest.py portable to Python 3
Lots of b prefixes here, and https://bugs.python.org/
issue29714 means
that this test is still very broken on Python 3.6 and 3.6.1, but 3.6.2
should things (based on testing using tip of the 3.6 branch from git).
#cleanup-only
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.