py3: add b'' prefixes in tests/test-extension.t
We are now close to getting this test pass on Python 3.
# skip-blame because just b'' prefixes.
Differential Revision: https://phab.mercurial-scm.org/D2478
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.