Merge branch 'selftests-mptcp-various-improvements'

Matthieu Baerts says:

====================
selftests: mptcp: various improvements

In this series from Geliang, there are various improvements in MPTCP
selftests: sharing code, doing actions the same way, colours, etc.

Patch 1 prints all error messages to stdout: what was done in almost all
other MPTCP selftests. This can be now easily changed later if needed.

Patch 2 makes sure the test counter is continuous in mptcp_connect.sh.

Patch 3 aligns the messages that are printed in mptcp_connect.sh.

Patch 4 prints each test results in mptcp_sockopt.sh, similar to what we
have in the TAP output.

Patch 5 moves the different test counters to a single one in
mptcp_lib.sh, to uniform how it is used.

Patch 6 moves how titles are printed from mptcp_join.sh to the lib, to
be reused in patch 7 by all other MPTCP selftests.

Patch 8 uses the '+=' operator to append strings instead of repeating
twice the variable name: that's shorter, easier to read.

Patch 9 adds colours for the [ OK ], [SKIP], [FAIL] and INFO keywords in
all MPTCP selftests.

Patch 10 to 12 are some preparation patches for patch 13: patch 10
modifies how some 'test_fail' helpers, patch 11 moves a helper from
userspace_pm.sh to the lib, and patch 12 changes where titles are
printed in userspace_pm.sh. Patch 13 moves some duplicated helpers from
mptcp_join.sh and userspace_pm.sh to mptcp_lib.sh.

Patch 14 moves duplicated read-only variables from mptcp_join.sh and
userspace_pm.sh to mptcp_lib.sh as well.

Patch 15 uses explicit variables instead of hard-coded numbers for the
exit status.
====================

Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-0-4f42c347b653@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>