Add registry profile designator tests (#243).

[skynavga]

Closes #243.

[nigelmegitt]

Looks good to me.

[cconcolato]

I agree the test matches the text in the spec:

All profile designators which have the TT Profile Namespace as a prefix but are otherwise not listed in Table 5-2 – Profiles or in [TTML Profile Registry] [...] must not appear in a conformant document instance.

But do we expect every validating implementation to have a copy of the profile registry?

[skynavga]

But do we expect every validating implementation to have a copy of the profile registry?

Most definitely not. Indeed, that is why we can only attempt a negative test, and, for that matter, we will need to update the registry to explicitly state that we will never define an entry called undefined.