TST: Benchmarks for CSV writing for various index variations
Benchmarks derived from GH37484 (https://github.com/pandas-dev/pandas/issues/37484). This captures a peculiar difference in timings between a DataFrame originating from `df.set_index(...).head(...)` and one from `df.head(...).set_index(...)`.

- [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

I've not included a whatsnew entry, as no section seems particularly relevant to internal-only test/benchmark changes. If it should go in, shout and I'll add a note somewhere.