Nice to have a few test vectors. In the longer term it might be nice to add tests for,
1. zero-length input
2. input exactly the same size as `BYTEARRAYSIZE`
but no hurry, needn't be in this PR.

(Though I wonder, if we do the JDK6 refactoring, a similar subclassing could happen in test classes, with only the test vector checksums differing. Someday...)