Merge pull request matplotlib#16735 from QuLogic/flaky-test_stem

TST: Make test_stem less flaky.

Author: tacaswell

lib/matplotlib/tests/test_axes.py

@@ -3342,10 +3342,15 @@ def test_hist_stacked_weighted():
     ax.hist((d1, d2), weights=(w1, w2), histtype="stepfilled", stacked=True)
 
 
-@pytest.mark.parametrize("use_line_collection", [True, False],
-                         ids=['w/ line collection', 'w/o line collection'])
-@image_comparison(['stem.png'], style='mpl20', remove_text=True)
-def test_stem(use_line_collection):
+@image_comparison(['stem.png', 'stem.png'], style='mpl20', remove_text=True)
+def test_stem():
+    # Note, we don't use @pytest.mark.parametrize, because in parallel this
+    # might cause one process result to overwrite another's.
+    for use_line_collection in [True, False]:
+        _test_stem(use_line_collection)
+
+
+def _test_stem(use_line_collection):
     x = np.linspace(0.1, 2 * np.pi, 100)
     args = (x, np.cos(x))
     # Label is a single space to force a legend to be drawn, but to avoid any