Replace the `Seed` value in the state proof light header with `BlockHash`.
It serves a similar purpose in terms of the second-order consideration
of defending against potential future hash-collision attacks (albeit a
little bit weaker), but has the benefit of allowing validating the entire
blockchain, by committing to the block hash itself in the state proof.

The test set a configurable consensus protocol, "test-fast-stateproofs",
and propagated the configuration to the child algod processes spawned
by the test, but did not update the supported consensus params in
the test process itself.  As a result, when the test code tried to
compute the light block header, it didn't have the block protocol
(test-fast-stateproofs) in its config.Consensus.