Disable tests when teachers logout & disable timeout for teacher/admin accounts

Fix for https://github.com/learningequality/ka-lite/issues/2403

We now disable exam mode automatically when a *teacher* (not an admin) logs out. This PR also disables the auto-logout due to timeout for teacher and admin accounts. 

Comes complete with tests @aronasorman !
Rather than implement the teacher-no-logging-out functionality as a decorator, why not put the logic in this if statement?