tests: add b'' prefixes to badserverext.py
This avoids a handful of failures due to missing str and bytes.
# skip-blame: just a bunch of b'' prefixes
Differential Revision: https://phab.mercurial-scm.org/D5750
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.