selftests: mptcp: adjust output alignment for more tests

The number of self tests in mptcp_join.sh will soon be more than 100, the
output alignment is no longer OK. This patch adjusted it.

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>