Test Repos for TTML{1,2}

After our last call, I gave some more thought to whether to use separate
repos for TTML{1,2} tests, and have come to the conclusion that this would
be a good idea, primarily as it allows separating spec issues from test
issues. While we could also do this by labeling, I think that overall it
will improve matters for editors and test maintainers. Therefore, unless
there is an objection, I would like Nigel or PLH to create the following
repos:

   - w3c/ttml1-tests
   - w3c/ttml2-tests

The current TTML1 tests would be migrated to the former, while TTML2 tests
would be populated into the latter.

G.