tests/helper-runtests.sh

rust-python-testing: separated base test classes

This will allow, e.g., to change `test-rust-discovery.py` simply
by adding the appropriate base class.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL