run-tests: fix Python 3 incompatibilities
At one point run-tests.py and test-run-tests.t worked and passed
under Python 3.5. Various changes to run-tests.py over the past
several months appear to have broken Python 3.5 compatibility.
This patch implements various fixes (all related to str/bytes type
coercion) to make run-tests.py and test-run-tests.t mostly work
again. There are still a few failures in test-run-tests.t due to
issues importing mercurial.* modules. But at least run-tests.py
seems to work under 3.5 again.