tests/helper-runtests.sh

byteify-strings: add support for ignore comments

Our simple token analysis is sometimes not clever enough, we need to be able
to turn off our script for parts of the code.

This change introduces three special comments:

    - `#no-py3-transform` to tell `byteify-strings` ignore the next line
    - `#py3-transform: off` to ignore everything until the end of the file
    - `#py3-transform: on` to stop ignoring

The last two can be particularly useful within Python 2/3 compatibility files.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL