run-tests: flush output stream before prompting to accept changes
With py3 on Windows, the prompt to accept changes in an --interactive run wasn't
showing up until after the choice was read from stdin.
py3: use util.forcebytestr() to convert error messages to bytes
This makes the python 3 buildbot green again.
Differential Revision: https://phab.mercurial-scm.org/D4811