Use subunit v2 for storage in file repository

[mtreinish]

Something that was in progress in testr before the fork was the migration to start using subunit v2 as the format for streams stored to disk in the file repository. We're using subunit v2 everywhere in testr except for storing it on disk. We convert it to v2 for all of the commands that read from the repository. I'm not sure how much work is actually needed to finish that migration, but it should simplify things if we can start using v2 everywhere.