selftests: mptcp: list sysctl data

Listing all the values linked to the MPTCP sysctl knobs was not
exercised in MPTCP test suite.

Let's do that to avoid any regressions, but also to have a kernel with a
debug kconfig verifying more assumptions. For the moment, we are not
interested by the output, only to avoid crashes and warnings.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20241021-net-mptcp-sched-lock-v1-3-637759cf061c@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>