[FOR DISCUSSION] [#75803512] Check for Cloudflare CF-RAY header

Add a flag to run tests specific to a vendor, and use that flag to support Cloudflare's `CF-RAY` header that is equivalent to `X-Served-By`.

__For discussion:__ If we're happy with this, I'll go ahead and feature-flag all the existing Fastly-specific tests to run only if `-vendor=fastly` is set.
Can we stick with one kind of string interpolation?