When creating fixtures, allow to pass the wiki

Up to now, create_test_cohort was hardwired to act on 'wiki'. This
made it cumbersome to populate 'wiki2'. Hence, we now extend the
function to allow to pass the target wiki. Thereby, the function can
be used to populate 'wiki2'.

Change-Id: I3a9973e39c50ec8f2814da79c2549544c46bdbf5