Merge branch 'selftests-mptcp-skip-tests-not-supported-by-old-kernels-part-1'

Matthieu Baerts says:

====================
selftests: mptcp: skip tests not supported by old kernels (part 1)

After a few years of increasing test coverage in the MPTCP selftests, we
realised [1] the last version of the selftests is supposed to run on old
kernels without issues.

Supporting older versions is not that easy for this MPTCP case: these
selftests are often validating the internals by checking packets that
are exchanged, when some MIB counters are incremented after some
actions, how connections are getting opened and closed in some cases,
etc. In other words, it is not limited to the socket interface between
the userspace and the kernelspace. In addition, the current selftests
run a lot of different sub-tests but the TAP13 protocol used in the
selftests don't support sub-tests: in other words, one failure in
sub-tests implies that the whole selftest is seen as failed at the end
because sub-tests are not tracked. It is then important to skip
sub-tests not supported by old kernels.

To minimise the modifications and reduce the complexity to support old
versions, the idea is to look at external signs and skip the whole
selftests or just some sub-tests before starting them.

This first part focuses on marking the different selftests as skipped
if MPTCP is not even supported. That's what is done in patches 2 to 8.
Patch 2/8 introduces a new file (mptcp_lib.sh) to be able to re-use some
helpers in the different selftests. The first MPTCP selftest has been
introduced in v5.6.

Patch 1/8 is a bit different but still linked: it modifies mptcp_join.sh
selftest not to use 'cmp --bytes' which is not supported by the BusyBox
implementation. It is apparently quite common to use BusyBox in CI
environments. This tool is needed for a subtest introduced in v6.1.

Link: https://lore.kernel.org/stable/CA+G9fYtDGpgT4dckXD-y-N92nqUxuvue_7AtDdBcHrbOMsDZLg@mail.gmail.com/
Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
====================

Link: https://lore.kernel.org/r/20230528-upstream-net-20230528-mptcp-selftests-support-old-kernels-part-1-v1-0-a32d85577fc6@tessares.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>