CLN: Remove unnecessary uses of pd. in tests
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

- Mostly edits along the lines of `pd.Series` -> `Series` for things that were already imported and had redundant uses of `pd`.  
- I cleaned some of the `import` statements, but I didn't import anything new, though that could be done in the future for further cleaning. 
- Was able to shorten some multi-line statements for a net deletion of ~100 lines without changing the content of any tests.
- Only did the base level `tests` and `tests/indexing` so far. 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=h1) Report
> Merging [#18277](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/69472f9ae9c645a0a5932c7c65b6efbdf66fbb5f?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18277/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18277      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49878    49878              
==========================================
- Hits        45590    45581       -9     
- Misses       4288     4297       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <ø> (ø)` | :arrow_up: |
| #single | `39.41% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18277?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/18277?src=pr&el=footer). Last update [69472f9...440e6ab](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=h1) Report
> Merging [#18277](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/63e8527d32aaf6afe1cd4b2a7b3bfadb088c9a72?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18277/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18277      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49880    49880              
==========================================
- Hits        45591    45583       -8     
- Misses       4289     4297       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <ø> (ø)` | :arrow_up: |
| #single | `39.42% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18277?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/18277?src=pr&el=footer). Last update [63e8527...92ddd5e](https://codecov.io/gh/pandas-dev/pandas/pull/18277?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jschendel just that one change is fine for now. the lint checks can be in a followup (or here if you can/want). @jreback : made the one change. not familiar with the lint checks, so I'll hold off until a followup. thanks @jschendel would love to come back with something that we can add to ``ci/lint.sh`` to check the codebase automatically.