Use OCR in locust tests

[hrj]

We are currently always testing with wrong answers. For proper testing, we need to submit answers that are known to be right or wrong.

For the tests to know what the right answer is, there are two possibilities:

1. We add a debug flag, which provides a hint in the challenge response. However, this requires us to put compromisable code in production path.
2. We add a DebugCaptchaProvider, which provides very simple challenges that are easily solved with OCR. This can be disabled by default, and a config.json can be provided by test scripts that enable this provider.

I prefer $2 because it means production code needn't have any hacks in it.