Extract expressions tests

I think a natural extension to the code extraction in serde would be to extract the tests to own files as well:
- #2019

Although the code extraction combined multiple expression from the same group into a single file I think for the tests each expression should have it's own file as there can be much more tests than implementations

## List
- [ ] Conditionals - https://github.com/apache/datafusion-comet/pull/2807
- [ ] `If`
- [ ] `CaseWhen`
- [ ] `Coalesce`
- [ ] Datetime
- [ ] `year`
- [ ] `month`
- [ ] `day_of_month`
- [ ] and more