selftests: mptcp: add fullmesh setting tests

This patch added the fullmesh setting and clearing selftests in
mptcp_join.sh.

Now we can set both backup and fullmesh flags, so avoid using the
words 'backup' and 'bkup'.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>