Split tests for response headers

These tests all inspect headers in the response from the CDN. They are
mostly for information and debugging purposes.

This commit is just a copy/paste with the imports adjusted appropriately.
I'll rename the tests separately.
Change description of Age header test

The expected behaviour for this changed during development when we found out
that the value in the Age header from origin will be incremented by edge if
the response is cached (has a CC header in this case).

Also moving the declaration of expectedAgeInSeconds to a constant at the top
of the test to make it look less magic.
Rename response header tests

So that they can be selected with `-run TestRespHeader`.

I've also given them much simpler names. I don't think it's necessary or
very easy to read the full description from the function name. The godoc
comment above should described the behaviour in plain English instead.
Merge pull request #59 from alphagov/split_resp_header_tests

Split response header tests