Selftests are supposed to run on any kernels, including the old ones not
supporting all MPTCP features.

Some tests are using IPTables and/or TC commands to force some
behaviours. If one of these commands fails -- likely because some
features are not available due to missing kernel config -- we should
intercept the error and skip the tests requiring these features.

Note that if we expect to have these features available and if
SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES env var is set to 1, the tests
will be marked as failed instead of skipped.

This patch also replaces the 'exit 1' by 'return 1' not to stop the
selftest in the middle without the conclusion if there is an issue with
NF or TC.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: 8d014eaa ("selftests: mptcp: add ADD_ADDR timeout test case")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>