tests/helper-runtests.sh

rust-cpython: do not convert warning pattern to utf-8 bytes

On Unix, both Rust Path and Mercurial expect a locale-dependent bytes,
and we don't support Windows yet.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL