runtest: also update slow test timeout during `#if` clauses For a `#if slow` test to be useful, we need the test timeout to be increased. Without this, the slow section would likely be killed before it finish.

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.

tests: increase timeout for slow test Test case `test-sparse-revlog.t` need some artifact (a bundle) build before it can run. The artifact is expensive to build, but can be reused from one run to the other. We are about to update that test to make the artifact building automatic if `--allow-slow-tests` is passed. However, we need a bump the timeout a bit to make sure the artifact building as time to finish. We could maybe teach run-tests.py how to directly handle such artifacts. However since there is only one of them for now, this seems premature. There are also some room to speed up the bundle creation for test-sparse-revlog.t