runner: Add an argument for test duration After the specified duration the runner stops executing new tests, but it doesn't interrupt running ones. Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Maria Kustova <maria.k@catit.be> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>