test/IstreamFilterTest: move pool_commit() calls to RAII destructor
test/IstreamFilterTest: move pool_commit() calls to RAII destructor

Make sure it runs after all other destructors.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy