tests/helper-runtests.sh

runtest: extract the logic that update timeout for slow tests

We want to use the "slow" test logic not only for "#require" clauses but also in
"#if" clauses. For this to be useful we need the logic to bump the timeout in at
least two spots. The first step it to factor it out.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL