Add tests

#### Added some serious test coverage for Opportunity form.
- [x] Test for saving an instance without any optional fields
- [x] Test for saving an instance with some optional fields (including an Other field)
- [x] Test that validation throws an error if a required field is mising
- [x] Test that **other** text fields are correctly parsed and saved
- [x] Test that **other** checkbox fields are correctly saved

@MrMaksimize please review!