[QueryTest] Restructure tests to allow easier extension

Summary:
The test documents are not touched by the individual tests, so there is
no need to set these up on every test iteration, only the DB has to
be set up and primed.

Preliminary change for further tests.

Test Plan:
ctest
(no functional changes)

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21862