DataFrame.truncate drops MultiIndex names
- [x] closes #34564
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
- [x] added tests
- [ ] passes tests (fixed for `DataFrame`, still issue for `Series`)
Thanks @arw2019 

Could you add a test to show how this fixes the linked issue? > Could you add a test to show how this fixes the linked issue?

@MarcoGorelli Added the test just now. It's currently inside `pandas/tests/indexing/multiindex/test_slice.py` but I'll move it if there's a better location for it!

As an aside, I looked in both `pandas/pandas/tests/indexes` and  `pandas/pandas/tests/indexing` and I couldn't find any other tests for `DataFrame.truncate`. Either here or in a separate pull request I'd be happy to add additional tests if people think it's a good idea! thanks @arw2019 