tests: closeable_test: mark count_stops::stop as noexcept
tests: closeable_test: mark count_stops::stop as noexcept

It never throws. This is now required by deferred_stop.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar