Add test coverage for Field API Number

We've had some recent WSODs and critical validation errors on core Field API widgets:

#557932: Taxonomy term field autocomplete widgets never validate, always lose values.
#557924: Options widget broken

Both of these widgets have things in common: they both use custom multiple values handling. They also have something in common with other D7 core Fields: very little test coverage. The only widgets with any tests in core are text_textfield and text_textarea.