Create automated test process

[martinohanlon]

Using pytest with mock or something else maybe running on the pico itself?

[martinohanlon]

I have made a proof of concept for running automated tests directly on the pico. It is in the tests branch.

https://github.com/RaspberryPiFoundation/picozero/tree/tests/tests

[martinohanlon]

I have refactored the poc to use unittest rather than my own "homegrown" test runner and written a significant number of tests for both input and output devices.

[martinohanlon]

I am going to merge the tests branch into dev. I have created tests for the main base devices (DigitalInput, DigitalOutput, AnalogInput, AnalogOutput, etc) and it seems to work and be helpful. Its getting difficult managing the 2 branches particularly when I find an issue when writing a test.

[martinohanlon]

I thought I could close this but test coverage is missing for Speaker and DistanceSensor :(