CDRIVER-2223: Parse integers for legacy JSON $date syntax

Since 933fd981, the JSON reader emitted an
error when encountering an integer value for a $date key. This restores the
previous behavior and allows legacy $date syntax to be parsed properly.
Consequently, a BSON corpus parse error test must now be skipped.

This commit adds previously absent tests for legacy $date syntax and adds
more valid and error cases to the existing $date $numberLong test.