Add Test Suite

[amouat]

I'd really like to see a test suite that runs the apko binary against various inputs and verifies the outputs.

I'm not sure if there's a tool to use for this - I'd probably just use a bash script myself, but I'm we might want something a bit more structured :)

A lot of this does get tested downstream, but the more we can catch upstream, the better.

The original impetus for this was I wanted somewhere to easily add a test for the changes in #690

[deitch]

I would be fine with something that just calls whatever func is called by the cli (whatever the cobra.command.RunE calls), so it is easier to write in go. But either way, exercising the entire chain would be a good thing.

We do have two types of full on build tests:

• we build everything in examples/ for one architecture, and one of them for all architectures
• we build and test several images, see stuff in hack/ci/, with thanks to @jdolitsky