Add a trivial prefix-based PasswordHasher fake and inject it into the
three test files that exercise CreateUser or AuthenticateUser. Drops
the full phpunit suite from ~7.4s to ~30ms (about 224x) without
losing coverage: the round-trip through hash/verify still validates
that CreateUser stores something other than the plaintext and that
AuthenticateUser only succeeds on a matching hash.
CreateUserTest is also refactored to use a setUp method, matching
the pattern already used in AuthenticateUserTest and AuthControllerTest.
