BUG: DataFrame.append with timedelta64
- [ ] 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

AFAICT several of the existing tests are just wrong, xref #39122 cc @jorisvandenbossche 

Fixes (but havent yet added a test for) https://github.com/pandas-dev/pandas/issues/39037#issuecomment-756748197

