Fix string format in test runner
`.format()` was expecting keyword arguments. Updated to match other skips nearby.

Hello @rhysparry! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/rhysparry/pandas/blob/80db0b25bc9f4b2d0eaed6a71170431677ad791f/pandas/util/testing.py) !

 - Complete extra results for this file :

> file_to_check.py:2442:-258: W605 invalid escape sequence '\('
file_to_check.py:2442:-255: W605 invalid escape sequence '\)'
---

 If you fetch and merge master the CI failures should be fixed. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=h1) Report
> Merging [#22598](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e2e1a1051576a48f210ce17272fc24b90ebcf24a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22598/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22598   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files         169      169           
  Lines       50782    50782           
=======================================
  Hits        46742    46742           
  Misses       4040     4040
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (ø)` | :arrow_up: |
| #single | `42.3% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22598/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.75% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=footer). Last update [e2e1a10...7525271](https://codecov.io/gh/pandas-dev/pandas/pull/22598?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @rhysparry : Thanks!