kyua: increase cleanup_timeout to 300s

Test cleanup times out after 60 seconds, causing the test to fail if the
cleanup hasn't finished in time. This timeout should be configurable,
but it currently is not.

Several tests (e.g. bridge, pf) are now reaching this limit; as a
workaround, increase the timeout to 300 seconds.

Reviewed by: igoro, kevans, des
Approved by: des (mentor), kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D50499