tests/helper-runtests.sh

rust: remove an unnecessary set of parentheses

My build complained about this. I guess it started after I upgraded
rustc.

Differential Revision: https://phab.mercurial-scm.org/D8020

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL