Run tests from previous version

[schultyy]

When there's a tests/ folder containing integration tests we check out the folder from the previous tag and run them against the current implementation.
If they fail and the user didn't specify a breaking change, the build fails.

[Ryman]

Testing downstream crates with cargo-crusader might also be something worth investigating, although probably only on CI since it executes random code.

[badboy]

Uh, that sounds like a good idea indeed. Thanks!