selftests: mptcp: pm netlink: fix shellcheck warnings

shellcheck recently helped to prevent issues. It is then good to fix the
other harmless issues in order to spot "real" ones later.

Here, two categories of warnings are now ignored:

- SC2317: Command appears to be unreachable. The cleanup() function is
  invoked indirectly via the EXIT trap.

- SC2086: Double quote to prevent globbing and word splitting. This is
  recommended, but the current usage is correct and there is no need to
  do all these modifications to be compliant with this rule.

For the modifications:

  - SC2034: ksft_skip appears unused.
  - SC2154: optstring is referenced but not assigned.
  - SC2006: Use $(...) notation instead of legacy backticks `...`.

Now this script is shellcheck (0.9.0) compliant. We can easily spot new
issues.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-10-bc79e6e5e6a0@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>