Use vendored check_figures_equal decorator function

**Description of the desired feature**

This is a medium to long term maintenance issue with the `check_figures_equal` function at `pygmt/helpers/testing.py`, which is pretty much an exact copy of [matplotlib's `check_figures_equal`](https://matplotlib.org/3.3.1/api/testing_api.html#matplotlib.testing.decorators.check_figures_equal) function. Ideally, we would just use a vendored function from `matplotlib` or `pytest-mpl` instead of writing our own.

> Yes, which was partly why I opened up the issue at https://github.com/matplotlib/pytest-mpl/issues/94, to get all of that `pytest-mpl` goodness (e.g. not having a hardcoded `result_dir`). I'll *try* to make a Pull Request to `pytest-mpl` for that, so we can just use a proper `@pytest.mark.mpl_check_equal` decorator in the future (will open a new issue after this one is merged). For now though, since we don't have many tests using `check_figures_equal` yet, we can probably just leave it like so.
>
> _Originally posted by @weiji14 in https://github.com/GenericMappingTools/pygmt/pull/555#discussion_r483292810_

**Are you willing to help implement and maintain this feature?** Yes, PR in the works at https://github.com/matplotlib/pytest-mpl/pull/95.