heptapod-ci: actually give pytest more time before timeout
`HGTEST_TIMEOUT` is overridden by `HGTEST_SLOWTIMEOUT` for tests marked as
slow, which `test-check-pytype.t` is. So this whole time the timeout was 1500s
(or 25 minutes), which is unfortunately not long enough for a *lot* of the
times it's run on the CI.
Differential Revision: https://phab.mercurial-scm.org/D11717