TST: Add test case when the key is a reserved key
- [ ] closes #24192 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I saw the reserved_key array on top of the `pandas/core/config.py` file.
```
_reserved_keys = ['all']  # keys which have a special meaning
```

So, I added the test case for test_register_option when the key is a reserved word, which is 'all'.

Please let me know if there is any issue. Thanks.

Hello @ywpark1! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/test_config.py`](https://github.com/ywpark1/pandas/blob/0c378b0f1023259de4c6093412eb3fe16d8106ea/pandas/tests/test_config.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=h1) Report
> Merging [#24193](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/77278ba8158457f142e6bba19e6d53a66a3f1322?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24193/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24193      +/-   ##
==========================================
+ Coverage   92.21%   92.21%   +<.01%     
==========================================
  Files         162      162              
  Lines       51723    51723              
==========================================
+ Hits        47695    47696       +1     
+ Misses       4028     4027       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43.03% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/24193/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `92.61% <0%> (-0.48%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24193/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.51% <0%> (+0.09%)` | :arrow_up: |
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/24193/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.7% <0%> (+0.66%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24193?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/24193?src=pr&el=footer). Last update [77278ba...0c378b0](https://codecov.io/gh/pandas-dev/pandas/pull/24193?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/24193?src=pr&el=h1) Report
> Merging [#24193](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/77278ba8158457f142e6bba19e6d53a66a3f1322?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24193/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24193      +/-   ##
==========================================
+ Coverage   92.21%   92.21%   +<.01%     
==========================================
  Files         162      162              
  Lines       51723    51723              
==========================================
+ Hits        47695    47696       +1     
+ Misses       4028     4027       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `43.03% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/24193/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `92.61% <0%> (-0.48%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24193/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.51% <0%> (+0.09%)` | :arrow_up: |
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/24193/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.7% <0%> (+0.66%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24193?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/24193?src=pr&el=footer). Last update [77278ba...0c378b0](https://codecov.io/gh/pandas-dev/pandas/pull/24193?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks, this is a duplicate test.