selftests: mptcp: set FAILING_LINKS in run_tests

Set FAILING_LINKS as an env var with a limited scope only when calling
run_tests().

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230623-send-net-next-20230623-v1-3-a883213c8ba9@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>