Split tests for request headers

These tests all inspect headers in a request received from edge by origin.

It leaves only a handful of tests left in the original uncategorised
"cdn_acceptance_test.go" file.
Rename request header tests

So that they can be selected with `-run TestReqHeader`.
Split remaining two tests to "misc"

The only two remaining tests are a bit miscellaneous in that they don't fit
into any of the existing categories and they don't necessarily deserve
categories of their own at the moment.
Rename misc tests

So that they can be selected with `-run TestMisc`.
Merge pull request #61 from alphagov/split_remaining_tests

[Fixes #14] Split remaining tests