TST: Debug flaky plotting test
Maybe closes https://github.com/pandas-dev/pandas/issues/19924. I can't reproduce this locally so who knows.

A while back I went through and added `plt.subplots()`  and explicit `ax=ax` to a bunch of the plotting tests. I apparently missed this one. It seemed to help, but they're flaky tests so who knows.

Also chanegd `idx1.equals(idx2)` to `tm.assert_frame_equal` so we get a better error message when it does fail.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19925?src=pr&el=h1) Report
> Merging [#19925](https://codecov.io/gh/pandas-dev/pandas/pull/19925?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f4c9d966fb20d9ca3221d31bd9eb31117065808c?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19925/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19925?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19925      +/-   ##
==========================================
+ Coverage   91.66%   91.69%   +0.02%     
==========================================
  Files         150      150              
  Lines       48974    48974              
==========================================
+ Hits        44894    44906      +12     
+ Misses       4080     4068      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.07% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.85% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19925?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19925/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19925?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/19925?src=pr&el=footer). Last update [f4c9d96...dd66dc5](https://codecov.io/gh/pandas-dev/pandas/pull/19925?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 It passed, not sure if it helped or not.

Merging later today if there are no objections.