tests/helper-runtests.sh

revset: remove redundant condition and change to else from elif

Before going to this piece of code, we already check whether n is in [0, 1, 2],
so dropping the redundant condition.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL