Enable "y_" JSON tests

[joeslice]

According to https://github.com/nst/JSONTestSuite, test cases whose filename begins with "y_" should result in no parse failure.

These tests were added in #19 but were left skipped to be addressed in this issue.

The scope of this issue is to:

• Enable "y_" tests and make them pass
• For any remaining skipped "y_" tests, file a follow-on issue to address them, or document why they are permanently skippable

[dmarcotte]

Closing this since we became "y_ compatible" in 175ac1b