Use module namespaces for shared tests

The names were getting pretty unwieldy, and were about to get even
crazier for the extracted string format tests et al.
Extract multipleOf/divisibleBy tests
Extract most string format tests

I originally also put `test_format_datetime` into the base shared format
tests, but our draft4 validation is significantly different from
draft1..3. I've not yet determined whether that is correct; draft4
passes through ruby's `DateTime.rfc3339` method, while the others are
validated against a regexp claiming to match ISO-8601 format.