[MTimeDB] Add some tests which include a 0 mtime

Summary:
Checks for D20315
Depends on D20317

Test Plan: ctest

Reviewers: Baloo, Frameworks, dfaure, ngraham, poboiko, astippich

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20318