datetime64 series and dataframe test GH#28273

- [ ] closes #28273 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

I used main codes to verify the GH28273. Unlike the original codes, the series and data frame types are same. So my codes just test its current behavior.

I have tested the ujson.{decode,encode} with np.datetime64. I can't find a method to decode the result of encode to datetime64. So I compare the decode and encode result directly. Actually, turns out this code doesn't exist in the current code base so it no longer needs testing. Closing but thanks for attempting to add a test for this