There is no realistic direct `storage_index_hash` "known answer"-style unit test

[exarkun]

allmydata.test.test_hashutil.HashUtilTest.test_known_answers attempts to provide certain hash-related regression detection by comparing the outputs of various hash functions for certain known inputs.

Since these hash functions are meant to be deterministic and stable over time, the value computed today should be the same as the value computed tomorrow.

The known input for the storage_index_hash test in this method uses a key of b"". However, all keys under real usage of this function must be 16 bytes long. It would be better if there were a direct test of this function for known input which could possibly be a real key instead of one which cannot possible be a real key.

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/948

[GitHub <noreply@…>]

In 08bb067/trunk:

Merge pull request #948 from LeastAuthority?/3575.realistic_storage_index_hash-key

Test storage_index_hash with a realistic key

Fixes: ticket:3575