Use vendored check_figures_equal decorator function

[weiji14]

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 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.