TST: adding test cases for verifying correct values shown by pivot_table() #21378
BUG: Incorrect values shown by pivot_table() #21378

- [x] closes #21378
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Just added a new test to verify the included patch in `master` branch