Add integration tests

[hannobraun]

The current unit tests are really not sufficient and a maintenance burden (I'm likely to remove them in an upcoming pull request).

Once embedded-hal is supported, an integration test suite should be added. This issue is currently blocked by #5.

[hannobraun]

I removed the unit tests in #12, as expected. This doesn't make the crate completely test-less though. There are some examples in the dwm1001 crate that serve as end-to-end tests.

[hannobraun]

This crate uses embedded-hal now, which means we can write integration tests and this issue is no longer blocked. I'm not going to tackle that right now though, as the crate is pretty well-tested via the aforementioned examples.

However, having a test suite that can be run without requiring a DWM1001-Dev board would be a great. If anyone were willing to pick this up, I'd appreciate it!