Rename response header tests
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.

mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests