Merge branch 'selftests-mptcp-refactoring-and-minor-fixes'

Mat Martineau says:

====================
selftests: mptcp: Refactoring and minor fixes

Patch 1 moves code around for clarity and improved code reuse.

Patch 2 makes use of new MPTCP info that consolidates MPTCP-level and
subflow-level information.

Patches 3-7 refactor code to favor limited-scope environment vars over
optional parameters.

Patch 8: typo fix
====================

Link: https://lore.kernel.org/r/20230623-send-net-next-20230623-v1-0-a883213c8ba9@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>