Add test with arbitrary

[da-x]

The test from #4 can be made accessible with -

cargo test --all-features

This PR adds the test. It still fails, however.

[chinedufn]

Some questions:

• How fast/slow are these tests?

• Do you have any examples of how other libraries make use of quickcheck + arbitrary? i.e. do they run it in CI, or do they just keep it behind a feature flag?

• Would you recommend that these run in CI?

• How do you see this evolving over time? What would a good arbitrary-based test suite look like? Are there more tests that you would want to see added over time?

• Do you know how/why this arbitrary test fails?

Thanks!