Add some unit tests to validate the length and stability of the xxhash interface

[rkoster]

As discussed at the FIPS retro it would be great if both sides of the buildpack lifecycle interface would have unit tests to validate the results of the xxhash library being used. So that in the future if for example the behaviour around padding changes a unit test would pick this up.

For reference here are the tests which are being run by the go side of the house: cloudfoundry/buildpackapplifecycle@f4b2bc9