When a required tool is missing, the return code 4 (SKIP) should be
returned instead of 1 (FAIL).

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 259a834f ("selftests: mptcp: functional tests for the userspace PM type")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>