Redesign check_figures_equal testing function to be more explicit

[weiji14]

Description of proposed changes

Improve readability of test code by explicitly declaring fig_ref and fig_test as pygmt.Figure objects, and returning the two figures to be compared against. Makes each testing code more standalone, though it will become 3 lines longer.

Before

@check_figures_equal()
def test_my_plotting_case(fig_ref, fig_test):
  "Test that my plotting function works"
  fig_ref.grdimage("@earth_relief_01d_g", projection="W120/15c", cmap="geo")
  fig_test.grdimage(grid, projection="W120/15c", cmap="geo")

After

@check_figures_equal()
def test_my_plotting_case():
  "Test that my plotting function works"
  fig_ref = pygmt.Figure()
  fig_ref.grdimage("@earth_relief_01d_g", projection="W120/15c", cmap="geo")
  fig_test = pygmt.Figure()
  fig_test.grdimage(grid, projection="W120/15c", cmap="geo")
  return fig_ref, fig_test

This PR is a small redesign of #555, and works towards #579 (vendoring the @check_figure_equal decorator to pytest-mpl.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.