Merge branch 'mptcp-selftests-refactor-join-tests'

Mat Martineau says:

====================
mptcp: selftests: Refactor join tests

The mptcp_join.sh selftest is the largest and most complex self test for
MPTCP, and it is frequently used by MPTCP developers to reproduce bugs
and verify fixes. As it grew in size and execution time, it became more
cumbersome to use.

These changes do some much-needed cleanup, and add developer-friendly
features to make it easier to see failures and run a subset of the tests
when verifying fixes.
====================

Link: https://lore.kernel.org/r/20220309191636.258232-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>