Fix failing munge_tabs test

This replaces all remaining occurrences of 'Open Ended Panel' with
'Assessment Panel'.

This was originally introduced in
cf95f7fa72e7eb597a2ca9359949827643bd585a

(cherry picked from commit 19f4de487ce3678d9a2738b77128155aebef672b)
Merge pull request #71 from Stanford-Online/stv/munge

Fix failing munge_tabs test