tests/helper-runtests.sh

py3: make dummyssh compatible with Python 3

Required encoding a str to bytes and writing bytes to a file
descriptor.

Differential Revision: https://phab.mercurial-scm.org/D2170

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL