Document the testbed/probe architecture

[freakboy3742]

What is the problem or limitation you are having?

We now have a test suite that is nearing 100% code coverage of the platform backends. However, the architecture of this testbed isn't obvious until you know what is going on. This is an impediment to contributors adding new features.

Describe the solution you'd like

We should add an architectural explainer about the test backend. This would be a background document, similar to the one explaining the "three layers" architecture.

Describe alternatives you've considered

No real alternative here, other than hoping contributors work out what is going on by osmosis.

Additional context

I recently wrote up a 10c version of this in response to someone asking how to test a new feature, and the very next ticket I responded to needed a link to that description. If that isn't a cry for documentation, I don't know what is.

[freakboy3742]

This probably doesn't need to be a completely separate document - it could be another section on the existing architecture guide. This should probably also cover what the core tests cover in comparison to what the testbed tests cover.

[mhsmith]

I think you've already done this at https://toga.readthedocs.io/en/latest/how-to/contribute-code.html#how-the-testbed-works.

[freakboy3742]

🤦

Thanks past Russell, you've saved me some work :-)