Added acceptance tests for enabling apps

This tests whether a user can see navigation entries after enabling
apps. This includes the app's group restriction.

This currently expects that a group "group1" exists until we have code
to auto-generate groups.

This commit also provides a utility function
Page.multiselectSetSelection() to make it possible to select entries
inside a multiselect.
Merge pull request #10844 from owncloud/acceptance-enableapps

Added acceptance tests for enabling apps