[libtaskmanager] Overhaul tasktools test

tasktools is complex code and deserves a good test.

This restructures and cleans up the existing test a bit to make it easier to follow and more extensible

Instead of doing everything in a custom temp dir rely on the directory provided by QStandardPaths::setTestModeEnabled

Make shouldFindApp data-driven

Create a virtual /usr/share/applications instead of manually populating some fields

Add new test for windowUrlFromMetadata