Test job cancel when scheduled on Docker

[wvengen]

Stopping jobs mostly works, but it has a number of cases to test.

1. Just created, but not running yet -> remove container without stopping it (not tested)
2. Running -> send signal (tested in PR Integration tests #21)
3. Finished -> do nothing (tested in PR Integration tests #21)

Can you think of more corner-cases?
Especially in the first case, there might be various stages.

See also documentation on scrapyd's cancel endpoint.

Note that in PR #21 tests have been added, including some for job cancellation. The main thing now is testing that a container is removed when a job in cancelled before it has started. This issue is now about implementing that, including finding a way to test it reliably.

[wvengen]

Related: #8.

[wvengen]

Basic testing added in PR #21. See #8 for things that could still be improved there.

[wvengen]

One approach to make this, is to add an option to create a Docker instance without starting it, e.g. when the schedule endpoint is called with a special query parameter (one that wouldn't be used as a setting), or perhaps a special header. Then the test can use it for testing this scenario.