[#76769514] Disable TestCacheVary for CloudFlare

CloudFlare don't currently support HTTP Vary as described by the RFC:

- http://tools.ietf.org/html/rfc7234#section-4.1

They have filed this as a feature request but don't yet have an
implementation date. So for now we'll have to disable this test when
`-vendor cloudflare` is specified.

This isn't ideal but since 513efe3 we do at least have some assurance that
`Accept-Encoding`, which is loosely related, is still working.