selftests: mptcp: connect: fail if nft supposed to work

In case of "external" errors when preparing the environment for the
TProxy tests, the subtests were marked as skipped.

This is fine but it means these errors are ignored. On MPTCP Public CI,
we do want to catch such issues and mark the selftest as failed if there
are such issues. We can then use mptcp_lib_fail_if_expected_feature()
helper that has been recently added to fail if needed.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368


Fixes: 5fb62e9c ("selftests: mptcp: add tproxy test case")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>