MDL-41555 behat: Fixing catch statement location

The exception is thrown when getting the
browser session, not when accessing the first
URL.
weekly release 2.5.2+
MDL-41315 assign: Dont show users not in the current grouping

This is for separate groups mode. Note - this will be slower - but
this feature is experimental and groupslib does not provide a nicer api.

Conflicts:

	mod/assign/locallib.php
MDL-41315 assign: Add unit test for groupmembersonly