Fail test when no vendor implementation known

Fail, rather than skip, a test if we haven't written test logic specific
to that vendor.

The aim of this is that new vendors will causes the tests to fail,
meaning that either:

a) the test should be modified to skip that test if the feature is
_known_ not be supported by the vendor, or,

b) the test should be modified to support the vendor's unique
implementation of the feature if it's supported, or,

c) the vendor CDN configuration needs modification to make the tests
pass

This will allow us to distinguish between features we know to be
unsupported, features that are supported but we don't yet test correctly
for, and features which have not yet been correctly configured in our
CDN account.

* * *

Also, fix a syntax error where `t.Skip()` was called as `t.skip()`.
Alphabetise constants