Improve keyutils tests

[josephlr]

The tests from libkeyutils are quite extensive and would be a good way to check that keyutils is properly exposing the kernel keyring API.

We should port these to be Rust integration tests. This will also make future changes like #24 much easier to make with confidence.

[mathstuf]

The core tests have been ported since #34. Keytype-specific tests would still be nice to have, but the uplift off of the library shouldn't care about that.