[Testing] Need integration tests or higher level tests

[julianstirling]

In playing round with return types I accidentally set all returns from ThingClient as None.

The only tests that errored were the ones for blobs. While the coverage is high, there probably needs to be another test suite with some more high level tests. Basically a simple simulation application which handles things like Things calling eachother, calling things over HTTP, something a bit more complex than in a unit test.

This will also help with checking that projects using LabThings can be typed.

[rwb27]

Absolutely. I guess we need a nicely-curated suite of example Things, and a set of things to do with them. The nicely-curated suite of example Things will then probably also come in handy in the unit tests, which currently have quite a bit of duplicated or nearly-duplicated example code in them (how many counter Things are in the codebase at present)...