Add message as constant for vendor-specific 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.

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