Add pre-hashing convergence tag construction unit tests

[exarkun]

Convergent encryption is achieved by using a convergence secret and a deterministic hashing and encryption protocol.

Since Tahoe correctly uses tags wherever it uses hashes, the hashing involved in this protocol also involves a unique tag.

This particular tag is dynamically constructed so that it isolates encryption results starting with different parameters from each other.

Add a direct unit test for the construction of this tag.

[GitHub <noreply@…>]

In 9ad4d31/trunk:

Merge pull request #968 from LeastAuthority?/3592.convergence-hasher-tag-tests

convergence hasher tag tests

Fixes: ticket:3592