selftests: mptcp: add extra_args in do_transfer

Instead of using a global variable mptcp_connect, this patch added
a new local variable extra_args in do_transfer() to store the extra
arguments passing to the mptcp_connect commands.

This patch also renamed the speed level 'least' to 'speed_*'. This
more flexible way can avoid the need to add new speed levels in the
future.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>