Implement TestNoCacheHeaderSetCookie() test

Send three requests. Origin keeps track of how many requests it has received
and adds a `Set-Cookie` header to each response. Then we assert that the
quantity of send and received requests are the same.

This may be improved to test response bodies in a subsequent commit.
Test resp bodies in TestNoCacheHeaderSetCookie()

To ensure that each response is unique.

Although not normal behaviour, there is no guarantee that edge won't send a
request to origin but still return a cached response. So checking the
quantity of requests received may not have been sufficient.
Merge pull request #20 from alphagov/origin_set_cookie

Implement TestNoCacheHeaderSetCookie() test