Add direct tests for FileHandle.get_encryption_key

[exarkun]

FileHandle?.get_encryption_key is responsible for deterministically and stably deriving an encryption key.

For the same inputs, it needs to derive the same key or convergent encryption becomes impossible.

At a test with well-known values in it to ensure a stable result.

[GitHub <noreply@…>]

In ab57746/trunk:

Merge pull request #969 from LeastAuthority?/3593.get_encryption_key-test

Add FileHandle?.get_encryption_key test

Fixes: ticket:3593