selftests: mptcp: uniform 'rndh' variable

The definition of 'rndh' was probably copied from one script to another
but some times, 'sec' was not defined, not used and/or not spelled
properly.

Here all the 'rndh' are now defined the same way.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>