Change a raw `Expect` test to poll asynchronously with `Eventually`

This test is flaky as it's only called once to check the value. It
needs to be polled over time as it can take up to a second for Redis
to acknowledge the write.
Merge pull request #68 from alphagov/change-direct-expects-test-to-be-async

Change a raw `Expect` test to poll asynchronously with `Eventually`