The problem is that 3d engines aren't required to output an exact image bit-for-bit. Some leeway is tolerated as long as the artifacts caused by it aren't visible to the viewer. A texture being 1% darker than expected is usually not a serious problem... usually. Under some conditions it might be a visual artifact. And that's the problem: It is hard to judge for an automated test which artifacts would be noticed by a human viewer and which are not. Still, automated tests can be used for simple sanity checks when used on very simple geometry.
What you could do is render some simple scenes and then compare the generated image with a reference rendering. That reference rendering could either come from another graphic engine (like your Ogre3d prototype) or be an earlier screenshots from the same engine (regression testing).
When some tests fail after a change to the graphic engine, a human tester should compare the output to the reference image and judge for themself if the new output looks just as good as before (or even better). When that's the case, the test should be changed to compare to the new, improved outputs.
Another thing you could check automatically are performance requirements. One such requirement could be that during a self-running demo (simulating a real sequence from the game) the frame-rate must not drop below 40 Fps on the test system. This is something you can test automatically. What you can't test is that the rendering is satisfying. But you can use such tests to prevent your developers from developing a game which looks superb but doesn't run properly on any affordable system until years after launch (hello Crytek).