Add unit test for definitionsForFileName() and definitionsForMimeType()

Summary: Add unit tests for newly added functions for KF 5.56.

Test Plan: make test

Reviewers: cullmann, davschul, vkrause

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D19286