selftests: mptcp: explicitly tests aggregate counters

Update the existing sockopt test-case to do some basic checks
on the newly added counters.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/385


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