[test] Add a test to check the priority of show and savefig

Since plt.savefig and plt.show do not work at the same time due to the
matplotlib design, we need to check whether show will not be called
when a figname is specified. We can actually raise an error, but plot
will be basically called in the end of an optimization, so I wanted
to avoid raising an error and just sticked to a check by tests.

test/test_utils/test_results_visualizer.py

@@ -55,15 +55,18 @@ def test_extract_dicts(cl_settings, with_ensemble):
 
 @pytest.mark.parametrize('params', (
     PlotSettingParams(show=True),
-    PlotSettingParams(show=False)
+    PlotSettingParams(show=False),
+    PlotSettingParams(show=True, figname='dummy')
 ))
 def test_plt_show_in_set_plot_args(params):  # TODO
     plt.show = MagicMock()
+    plt.savefig = MagicMock()
     _, ax = plt.subplots(nrows=1, ncols=1)
     viz = ResultsVisualizer()
 
     viz._set_plot_args(ax, params)
-    assert plt.show._mock_called == params.show
+    # if figname is not None, show will not be called. (due to the matplotlib design)
+    assert plt.show._mock_called == (params.figname is None and params.show)
     plt.close()