BUG: fix `groupby.quantile` inconsistency when interpolation='nearest'
I removed the xfail mark in previous tests. Do we need to add more tests?
Under the hood, `np.quantile` uses `np.round` for nearest interpolation, which rounds to the even idx when exactly halfway.

- [x] closes #47942 (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
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @yuanx749 