Implement TestNoCacheHeaderSetCookie() test
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.

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