Adds the unicode-org/message-format-wg repo as a submodule at test/messageformat-wg, and uses its upstream test suite.

To reduce the number of files brought in, using a sparse checkout may be a good idea:

git submodule update --init
cd test/messageformat-wg
git sparse-checkout set test

This PR depends on unicode-org/message-format-wg#826 first fixing a few test errors that were identified in the process.

A few error code updates and other minor bugfixes are included as they were discovered during these updates.