Cleanup tests: remove old code, fix user fixtures, etc

### WHAT

A cleanup of tests. Mostly the cleanup from https://github.com/binarylogic/authlogic/pull/421, without the crazy Rails 4.1 config fixes.

### WHY

Breaking this apart from https://github.com/binarylogic/authlogic/pull/421 so I can work on another solution to fix Rails 4.1 tests.
The legacy user fixtures were created with the old default algorithm hashing (`Sha512`), so let's just treat them like the legacy users that they are.