Make mock available by default

- The require will not trigger until redis_mock accessed.

- If we happen to call `SidekiqUniqueJobs.redis_mock.flushdb`
  in a `before(:each)` but `testing_enabled?` hasn't yet been
  called, tests will all blow up. This avoids the need for an
  extra `require` in the test/spec helper.

- Switches to ruby's built-in `defined?` operator.