Safe version of ascii macros, add missing tolower_ascii
- [x] fixes a warning introduced in #23981
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Using unparenthesized macro parameters is dangerous, so wrap `c` in braces.

Example warning fixed by adding `tolower_ascii()`:

```
In file included from pandas/_libs/lib.c:643:0:
pandas/_libs/src/parse_helper.h: In function ‘lowercase’:
pandas/_libs/src/parse_helper.h:141:26: warning: implicit declaration of function ‘tolower_ascii’; did you mean ‘toupper_ascii’? [-Wimplicit-function-declaration]
     for (; *p; ++p) *p = tolower_ascii(*p);
                          ^~~~~~~~~~~~~
                          toupper_ascii
```
lgtm - thanks! I did this originally, I always forget the right wrapping to make macros safe(er)
 @jreback  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25836?src=pr&el=h1) Report
> Merging [#25836](https://codecov.io/gh/pandas-dev/pandas/pull/25836?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f2bcb9f0060d030085cdbaeeb638664c6f73e636?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25836/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25836?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25836      +/-   ##
==========================================
- Coverage    91.3%    91.3%   -0.01%     
==========================================
  Files         173      173              
  Lines       53004    53004              
==========================================
- Hits        48397    48396       -1     
- Misses       4607     4608       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <ø> (ø)` | :arrow_up: |
| #single | `41.77% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25836?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25836/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25836?src=pr&el=footer). Last update [f2bcb9f...0d75c62](https://codecov.io/gh/pandas-dev/pandas/pull/25836?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @vnlitvin 