tests/helper-runtests.sh

rust-cpython: moved generic conversion fn out of ancestors module

This will allow to use it easily from other submodules

Differential Revision: https://phab.mercurial-scm.org/D5578

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL