TST: split tests/format/test_format.py
closes #15531
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15546?src=pr&el=h1) Report
> Merging [#15546](https://codecov.io/gh/pandas-dev/pandas/pull/15546?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/29d81f3df81eb0a4d077ae1317df74d509cdc446?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15546      +/-   ##
==========================================
- Coverage   91.06%   91.04%   -0.03%     
==========================================
  Files         136      136              
  Lines       49104    49104              
==========================================
- Hits        44718    44706      -12     
- Misses       4386     4398      +12
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15546?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/29d81f3df81eb0a4d077ae1317df74d509cdc446...4c5ef4cfa0556d1680a9c9163aa347a13a9d6590?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `40% <0%> (-46.67%)` | :x: |
| [pandas/util/decorators.py](https://codecov.io/gh/pandas-dev/pandas/compare/29d81f3df81eb0a4d077ae1317df74d509cdc446...4c5ef4cfa0556d1680a9c9163aa347a13a9d6590?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvZGVjb3JhdG9ycy5weQ==) | `67.92% <0%> (-0.95%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/29d81f3df81eb0a4d077ae1317df74d509cdc446...4c5ef4cfa0556d1680a9c9163aa347a13a9d6590?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.78% <0%> (-0.35%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/29d81f3df81eb0a4d077ae1317df74d509cdc446...4c5ef4cfa0556d1680a9c9163aa347a13a9d6590?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15546?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15546?src=pr&el=footer). Last update [29d81f3...4c5ef4c](https://codecov.io/gh/pandas-dev/pandas/compare/29d81f3df81eb0a4d077ae1317df74d509cdc446...4c5ef4cfa0556d1680a9c9163aa347a13a9d6590?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).