Add a test case for useful POW

[PaulGustafson]

Idea: make "algo" a map from a fixed parameter space to a fixed function space
Use the hash as a seed for a dataset generator (or dataset constraint)
Hash -> Dataset (preagreed)
Acceptance criterion (function with at most specified loss)
BTC : ByteString must have specified format (from tx pool) except for nonce
Acceptance criterion: n-projection to 0 has 0 loss