TST: reduce amount of nesting in tests, specifically move core routines higher
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16030?src=pr&el=h1) Report
> Merging [#16030](https://codecov.io/gh/pandas-dev/pandas/pull/16030?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0e2bbcf95624e5312c9ba0f9de48e9b2a1f2ede0?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/16030/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16030?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16030      +/-   ##
==========================================
- Coverage      91%   90.99%   -0.01%     
==========================================
  Files         153      153              
  Lines       50469    50469              
==========================================
- Hits        45927    45924       -3     
- Misses       4542     4545       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16030?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16030?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16030?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.55% <0%> (-0.19%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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