Move PurgeAllKeys func into helpers test file for reuse

We're going to be purging all keys from Redis whenever we build up or
use a Redis service (like ttl_hash_set objects). Share this code for
easier reuse in `AfterEach` blocks.
[#73135254] HealthCheck function returns the status of Redis and RabbitMQ

This is wrapped up by a Status object so that we can pretty-print the
keys into JSON for use by an HTTP HandlerFunc later.
[#73135254] HealthCheck is an object that can return an HTTP Handler

To build out our HTTP health check endpoint we're going to convert our
simple function into an object with multiple methods attached. These
methods return the Status() and an HTTPHandler(). The latter can be
used as part of Go's HTTP server and attached using
http.HandlerFunc().

This is wired into the main function and the port can be configured
using environment variables.
Merge pull request #13 from alphagov/73135254-http-health-check

[#73135254] HTTP health check