TST: register slow marker
Closes https://github.com/pandas-dev/pandas/issues/16680
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16797?src=pr&el=h1) Report
> Merging [#16797](https://codecov.io/gh/pandas-dev/pandas/pull/16797?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/664348c440ccb4dab3d4c420d6aaee7c688c9b0e?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/16797/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16797?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16797   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         161      161           
  Lines       49260    49260           
=======================================
  Hits        44797    44797           
  Misses       4463     4463
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.71% <ø> (ø)` | :arrow_up: |
| #single | `40.18% <ø> (ø)` | :arrow_up: |


------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16797?src=pr&el=footer). Last update [664348c...72328ef](https://codecov.io/gh/pandas-dev/pandas/pull/16797?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
