Add unittest to check if commiting the same change twice works

Summary:
The fileindexer is only aware some terms/properties *may* have changed,
but may also submit a document again which is identical to the current
DB contents.
Ideally, the engine should catch these cases and remove any noops from
the transaction.

Test Plan: ctest

Reviewers: Baloo, Frameworks, ngraham, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D17199