Feature-flag tests not supported by Cloudflare

These tests will be skipped; Cloudflare doesn't use the `X-Cache` or
`X-Cache-Hits` header.

It does have a `CF-Cache-Status` header which is
equivalent to `X-Cache`, but this will not append to the `X-Cache`
header coming from origin.
Add support for Cloudflare's CF-Cache-Status

Minimum viable change to support Cloudflare's `CF-Cache-Status` header,
which is equivalent to `X-Cache`. This currently only checks for a
cache `MISS`.
Test for header containing 'HIT' on cache hit
Refactor CacheHitMiss test to use a loop

Ready for extending this test to support Cloudflare's 'EXPIRED' status
in the next commit.
Support Cloudflare's 'EXPIRED' status

Cloudflare's `CF-Cache-Status` header may contain one of the
following[1]:
  - HIT
  - MISS
  - EXPIRED
  - STALE
  - IGNORED
  - REVALIDATED

We already support `HIT` and `MISS`; this commit adds support for
`EXPIRED`, which is used when a previously-requested object is requested
one further time after its TTL. We specify the object's TTL using the
`Cache-Control` `max-age` header.

I haven't implemented `STALE`, `IGNORED` or `REVALIDATED` because they
only occur under very specific circumstances and so are less useful to
us.

Note that I expected to see `IGNORE` rather than `MISS` on an object's
first request, as per Cloudflare's documentation, but in reality the
`IGNORE` status did not occur in my testing.

[1]:
https://support.cloudflare.com/hc/en-us/articles/200168266-What-do-the-various-CloudFlare-cache-responses-HIT-Expired-etc-mean-
Clarify what 'EXPIRED' is

...by creating a new array called `cloudFlareStatuses`.
Verify status received after 'EXPIRED'

Extend the `CacheHitMiss` test to document what status we see from
Cloudflare after `EXPIRED`.

`EXPIRED` indicates that the object was cached but has past its TTL and
therefore a new request is made to origin. It makes sense then that the
next request after `EXPIRED` should be a `HIT`.
Merge pull request #94 from alphagov/75803332-x-cache-cloudflare

[#75803332] Add support for Cloudflare's CF-Cache-Status