selftests: mptcp: add tc check for check_tools

tc are used in some test scripts: mptcp_connect.sh, mptcp_join.sh and
simult_flows.sh. It makes sense to check if tc is installed before running
these scripts, just like other tools. So this patch add 'tc' check for
mptcp_lib_check_tools(), and check it in these test scripts.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>