tests/helper-runtests.sh

rust-index: avoid some cloning in find_gca_candidates()

Instead of keeping the information whether the current revision is
poisoned on `current_seen`, we extract it as a boolean.

This also allows us to simplify the explanation of `seen[r].is_poisoned()`,
as the exceptional case where it is poisoned right after `r` has been
determined to be a solution does no longer exist.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL