Testing mode for hypothesis explain phase

[asmeurer]

Hypothesis has a new feature that guess what line of code caused a failure https://hypothesis.readthedocs.io/en/latest/settings.html#phases

We should add some option to enable it in the tests. Currently it is disabled by default. But even if you enable it with --hypothesis-explain, it doesn't work, because coverage has to be disabled. We should also disable the explicit examples, because they prevent it from working (which maybe should be considered a hypothesis bug).