[Bookmarks Runner] Cleanup tests CMakeList

Summary:
Build the test dependencies as static library. Remove duplicate and
commented out lines. Use ecm_add_test instead of setting up everything
manually.

Test Plan: make && ctest

Reviewers: #frameworks, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16404