Bug 1054326: Initial attempt to submit Imagecompare test framework and test cases int...

...o the main branch
By the way, can you try taking a screenshot using the following code, it should be identical to the image you're getting at the moment but without the need to simulate hardware events and pull images from the device:
```python
marionette.set_context(marionette.CONTEXT_CHROME)
screenshot = marionette.screenshot()
marionette.set_context(marionette.CONTEXT_CONTENT)
```