Implement board tests

[lkijanovic]

Essentially, the idea is to have a set of hardware tests that should be kept simple and try to isolate a module/functionality as much as possible.
Some of the immediate and eventual benefits of this:

• New features can be tested (relatively) isolated before integrating them in the main app
• When the moment comes (and it will come) that you stop trusting HW, there will be a reference test to check whether you're losing your mind
• During the bring-up of future boards, tests will be a useful starting point

I would probably suggest placing these in src/tests/hardware/, at least in the first iteration.

[DNedic]

This would be nice to have. We could also have a physical machine with boards attached to it to run CI tests on real hardware.

With the current architecture of the repo, the repo itself represents a freestanding app and anything inside src is a part of the app, so having other apps in there would not make sense IMO. We could create a new repo for test apps, or rearchitect the repo folder structure (but try preserving git blame).

[filipembedded]

What about an idea of placing those tests in efc/demos/some_demo where main application will be placed, as it is now, inside efc/app. I have tried this on feat/esc branch with some demo that I wrote for escs, and it worked fine. This way is handy as it doesn't require additional repository, and every demo that user needs is in demos directory.