[#75802710] Improve existing Age resp header test

By using a loop to repeat actions and assertions. This now asserts the
response header from the first request. It also avoids an unnecessary
strconv operation. Has been renamed to indicate that origin is setting the
first response header.
[#75802710] Test Age header when not set by origin

This compliments the existing TestRespHeaderAgeFromOrigin test and will help
us clarify how much CloudFlare have implemented.

The first value of "0" passes for Fastly and Varnish, but fails for
CloudFlare who return no header on the first response. The RFC isn't clear
which is the right behaviour but I'm leaning towards the former.
Merge pull request #111 from alphagov/75802710-improve_age_resp_header_tests

[#75802710] Improve and extend Age response header tests