BUG: setting td64 value into numeric Series incorrectly casting to int
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

We fixed this a few days ago for some of the setting-methods, but there were a few we missed.  Re-using the TestSetitemEquivalents pattern is much more thorough.

We'll be able to de-duplicate the test class following #39477