Makes testing easier and reduces dependency on strong random sources.

We only blind outputs when we have an unlocked wallet currently (I think) so we could just do a tagged hash of the commitment and the master wallet blinding key.

Unsure what we should use for the blinding factors themselves. A hash of the scriptPubKey + amount could work, but would result in equal commitments when people reused addresses, which we ought not to compromise privacy on even more..