use tm.assert_equal instead of parametrizing assert funcs
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/indexes/datetimes/test_tools.py`](https://github.com/jbrockmendel/pandas/blob/306e940f6852781371dd1c42aaec26cbb7304b65/pandas/tests/indexes/datetimes/test_tools.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/test_nat.py`](https://github.com/jbrockmendel/pandas/blob/306e940f6852781371dd1c42aaec26cbb7304b65/pandas/tests/scalar/test_nat.py) !

 - There are no PEP8 issues in the file [`pandas/tests/tseries/offsets/test_offsets.py`](https://github.com/jbrockmendel/pandas/blob/306e940f6852781371dd1c42aaec26cbb7304b65/pandas/tests/tseries/offsets/test_offsets.py) !

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/jbrockmendel/pandas/blob/306e940f6852781371dd1c42aaec26cbb7304b65/pandas/util/testing.py) !

 - Complete extra results for this file :

> file_to_check.py:2453:-258: W605 invalid escape sequence '\('
file_to_check.py:2453:-255: W605 invalid escape sequence '\)'
---

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22995?src=pr&el=h1) Report
> Merging [#22995](https://codecov.io/gh/pandas-dev/pandas/pull/22995?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d430195ffaa37df2d8fc3ecf26306bb5e06b4ad0?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22995/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22995?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22995      +/-   ##
==========================================
- Coverage    92.2%   92.19%   -0.01%     
==========================================
  Files         169      169              
  Lines       50822    50826       +4     
==========================================
+ Hits        46858    46861       +3     
- Misses       3964     3965       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <75%> (-0.01%)` | :arrow_down: |
| #single | `42.35% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22995?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22995/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `86.18% <75%> (-0.05%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22995?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/22995?src=pr&el=footer). Last update [d430195...306e940](https://codecov.io/gh/pandas-dev/pandas/pull/22995?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
