TST: better testing of Series.nlargest/nsmallest
xref #15299 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=h1) Report
> Merging [#15902](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0a3706780feb77f241715ffcdebb14ad7d678d3d?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15902/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15902      +/-   ##
==========================================
- Coverage   90.97%   90.96%   -0.01%     
==========================================
  Files         145      145              
  Lines       49496    49513      +17     
==========================================
+ Hits        45027    45039      +12     
- Misses       4469     4474       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.72% <75%> (-0.01%)` | :arrow_down: |
| #single | `40.62% <4.16%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `93.69% <75%> (-0.75%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15902?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/15902?src=pr&el=footer). Last update [0a37067...657eac8](https://codecov.io/gh/pandas-dev/pandas/pull/15902?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).