[#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`.

I have also feature-flagged the existing Fastly-specific tests to run only if `-vendor=fastly` is set.

I also now intentionally cause a test to fail if it has not been adapted for a specified vendor. If a feature is known not to be supported by a vendor, we skip the test. This means that adding new CDN vendors will show failures until the tests are adapted for the new vendor (or the configuration at vendor is corrected).

This also means we're now specifying Fastly as the vendor when we run the mock tests; this is because the Varnish behaviour as-is closely mimics Fastly. We can look at mimicking other vendors using the Varnish mock instance in future.
Is this necessary? It duplicates the documentation for the mocks further down.