TST: fix warnings on multiple subplots
- [ ] xref #37178 partially
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This PR suggests a (partial) fix for such warnings, which take place when running test suite on ``pandas/tests/plotting``.
```
UserWarning: To output multiple subplots, the figure containing the passed axes is being cleared
```

The problem is the following.
The function ``_check_plot_works`` creates subplot(211) if axes are not provided.
This is fine, when the plot to be created is a single-axes plot.
However, if the plot itself is to be plotted on multiple axes (for instance, df.hist would plot each column on a dedicated subplot, by default), then the warnings mentioned are emitted.

Two ways to handle it:
- Find cases where plot is a multiaxes-plot and catch warnings there. This is fine, but in some parametrized tests the plots are either single-axes or multiple-axes. It makes it difficult to figure out without splitting the tests.
- Change function ``_check_plot_works``: do not create subplot(211).

I created ``_check_single_plot_works`` which does not create subplots and used it in the problematic cases.
I guess, that the name is not the best (I would prefer ``_check_plot_works``), but I encourage those concerned to have a discussion here.

Please see comments in ``_check_plot_works`` (I will remove them once we come to the conclusion on the topic).