runtest: move slow timeout process earlier in the `_hghave` method Before this changesets, early returns might prevent this logic to apply. It seem safer to run it sooner.

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.