ASV: add benchmarks for DataFrame.xs with a MultiIndex
- [x] closes #35188 
- [x] [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).

The slowdown in the linked issue has already been resolved. This PR just adds some ASVs.
thanks @lukemanley 