Fix KBoomarkMenuTest when Bookmarks Editor is not installed

Summary:
Fix KBoomarkMenuTest when Bookmarks Editor is not installed. The test
depended on the Bookmarks Editor being installed.

Reviewers: dfaure, aacid, ngraham

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20860