Add flag to specify vendor-specific tests to run

Different vendors supply different headers, so we need to vary the tests
accordingly.

Add a new command-line flag, `-vendor`, which takes a string denoting
the name of the vendor against which we're running the tests.
Add message as constant for vendor-specific tests

When a vendor-specific test does not apply to the vendor specified, we
need to skip that test. Add a message as a constant so that we can
re-use it as necessary.
Repurpose X-Served-By test to be generic

Repurpose the `X-Served-By` test such that it supports multiple vendors,
not just Fastly.

Also, add support for Cloudflare's `CF-RAY` header.
Rename `testForFastly` to `vendorFastly` etc

To make it clearer that we are referring to vendors.