Parallelize tests based on test case

[Rickkwa]

So matrix based on test case file. That way, we can increase parallelism and reduce speed of tests. Otherwise, every time we add a new test case file, the tests will take longer.

[Rickkwa]

And can we build the images just once instead of every job to save time? ie. like a build stage to build the images and pass them to the subsequent stages. I don't want to use dockerhub as a dependency.