Investigate adding property-based testing?

[pfmoore]

I think it could be useful to add some property-based tests to resolvelib, to stress test some of the basic properties of the resolver. Some ideas I had for useful tests:

• If the resolver succeeds, the resolver result will satisfy the supplied requirements.
• Given a set of requirements, adding an extra one that contradicts one in the set will cause a resolver failure.

The key will be setting up "strategies" that generate sets of candidates and requirements.

I'm mainly adding this here as a reminder, so that I don't forget to look at this at some point in the future.