TST: fix flaky test
One of the tests that uses this fixture modifies its value, so the `scope` kwarg is incorrect.  The failure depends on the order in which the test is run.  AFAICT only showing up on OSX.

Hits one of the checkboxes in #26823.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=h1) Report
> Merging [#27004](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4d4ec5a36acda40f13a8c3c3e19262c095d4c41?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/27004/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #27004      +/-   ##
==========================================
- Coverage   91.99%   91.99%   -0.01%     
==========================================
  Files         180      180              
  Lines       50774    50774              
==========================================
- Hits        46712    46708       -4     
- Misses       4062     4066       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `41.85% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/27004/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/27004/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27004?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/27004?src=pr&el=footer). Last update [b4d4ec5...6f2acdf](https://codecov.io/gh/pandas-dev/pandas/pull/27004?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/27004?src=pr&el=h1) Report
> Merging [#27004](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4d4ec5a36acda40f13a8c3c3e19262c095d4c41?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/27004/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #27004      +/-   ##
==========================================
- Coverage   91.99%   91.99%   -0.01%     
==========================================
  Files         180      180              
  Lines       50774    50774              
==========================================
- Hits        46712    46708       -4     
- Misses       4062     4066       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `41.85% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/27004/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/27004/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27004?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/27004?src=pr&el=footer). Last update [b4d4ec5...6f2acdf](https://codecov.io/gh/pandas-dev/pandas/pull/27004?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jbrockmendel 