Implement all NoCache tests

Implement all NoCache tests for standard CDN behaviour. Includes a helper and a mock (see commit for details).

These have uncovered a "bug" probably caused by us returning too early in `vcl_recv`. I'll raise something in Pivotal to fix that in our service configs.
```
--- FAIL: TestNoCacheHeaderAuthorization (0.08 seconds)
        helpers.go:130: Incorrect response body. Expected "second response", got "first response"
        helpers.go:130: Incorrect response body. Expected "third response", got "first response"
=== RUN TestNoCacheHeaderCookie
--- FAIL: TestNoCacheHeaderCookie (0.07 seconds)
        helpers.go:130: Incorrect response body. Expected "second response", got "first response"
        helpers.go:130: Incorrect response body. Expected "third response", got "first response"
```