CDRIVER-3970: Remove DBRef validation from Extended JSON parser (#800)

* CDRIVER-3970: Remove DBRef validation from Extended JSON parser

Documents that resemble but do not conform to a DBRef should be left as-is.

Syncs BSON corpus tests with mongodb/specifications@a274fbfddfd39f5f3d00712c422e4f1b062d783c

Skips were added for additional corpus tests unrelated to this spec change.

Use hex escaping instead of unicode for MSVC compatibility