TST: Enable tests that aren't currently running in indexes/datetimes/test_tools.py
- [X] closes #17403
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Enabled tests that aren't currently running.  Made a small modification to a typo in `datetimes.py` to make sure all tests pass.  Small modifications to checks in `TestToDatetimeUnit::test_frame` to verify that the raised messages match expectations.


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=h1) Report
> Merging [#17405](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/062f6f118fe4ea439ae255a8ff886a532e20ecdb?src=pr&el=desc) will **increase** coverage by `0.11%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17405/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17405      +/-   ##
==========================================
+ Coverage   91.03%   91.14%   +0.11%     
==========================================
  Files         163      163              
  Lines       49580    49580              
==========================================
+ Hits        45137    45192      +55     
+ Misses       4443     4388      -55
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.93% <ø> (+0.12%)` | :arrow_up: |
| #single | `40.26% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `85.43% <ø> (+18.4%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.43% <0%> (+0.09%)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.29% <0%> (+0.44%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17405?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/17405?src=pr&el=footer). Last update [062f6f1...a09ac59](https://codecov.io/gh/pandas-dev/pandas/pull/17405?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @jschendel!