baloo-widgets: Create test to assert metaDataRequestFinished is emitted once only
Summary: CCBUG: 388583
Test Plan: Old code fails this test
Reviewers: elvisangelaccio, smithjd, vhanda, ngraham, Dolphin
Reviewed By: elvisangelaccio, Dolphin
Differential Revision: https://phabricator.kde.org/D10119