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.
can you show these your PR (e.g. `asv dev ` just see the timings Yes, here's the benchmark output:

```
$ asv run -e -E existing -b ^io.csv.ToCSVIndexes
· Discovering benchmarks
· Running 3 total benchmarks (1 commits * 1 environments * 3 benchmarks)
[  0.00%] ·· Benchmarking existing-py_home_jbanorthwest.co.uk_danielevans_venvs_pandas_dev_py39_bin_python3.9
[ 16.67%] ··· Running (io.csv.ToCSVIndexes.time_head_of_multiindex--)...
[ 66.67%] ··· io.csv.ToCSVIndexes.time_head_of_multiindex                                                  2.04±0.04s
[ 83.33%] ··· io.csv.ToCSVIndexes.time_multiindex                                                            818±60ms
[100.00%] ··· io.csv.ToCSVIndexes.time_standard_index                                                        645±60ms
``` Hello @DanielFEvans! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-01-21 16:25:08 UTC thanks @DanielFEvans 