TST: Use __tracebackhide__ to suppress unnecessary parts of tm assertions
Makes test output much more to the point.

Also gets rid of some backslashes, both for prettiness and to get rid of deprecation warnings in test logs.
Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/util/testing.py`](https://github.com/jbrockmendel/pandas/blob/547f2f731fa91716035eed009c3b6137e6af9af3/pandas/util/testing.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23307?src=pr&el=h1) Report
> Merging [#23307](https://codecov.io/gh/pandas-dev/pandas/pull/23307?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ff55b3236234d771c1d499304cf8ac1bc0eacc0c?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23307/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23307?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23307      +/-   ##
==========================================
+ Coverage   92.22%   92.22%   +<.01%     
==========================================
  Files         169      169              
  Lines       50889    50897       +8     
==========================================
+ Hits        46932    46940       +8     
  Misses       3957     3957
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.65% <100%> (ø)` | :arrow_up: |
| #single | `42.29% <64.28%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23307?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/23307/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `86.61% <100%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23307?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/23307?src=pr&el=footer). Last update [ff55b32...547f2f7](https://codecov.io/gh/pandas-dev/pandas/pull/23307?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!