selftests: mptcp: centralize stats dumping

If a test case fails, the mptcp_join.sh script can dump the
netns MIBs multiple times, leading to confusing output.

Let's dump such info only once per test-case, when needed.
This additionally allow removing some code duplication.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>