TST: force test with shared test image to run in serial

The tests test_hexbin_empty and test_hexbin_log_empty use the same target
image, however if they the tests ended up being run in different workers when
running the tests in parallel they will step on each other if one of the test
starts writing the output file while the other is trying to read the test
image.

By putting them in the same test they will save and evaluate in order.

Author: tacaswell

lib/matplotlib/tests/test_axes.py

@@ -894,18 +894,14 @@ def test_hexbin_extent():
     ax.hexbin("x", "y", extent=[.1, .3, .6, .7], data=data)
 
 
-@image_comparison(['hexbin_empty.png'], remove_text=True)
+@image_comparison(['hexbin_empty.png', 'hexbin_empty.png'], remove_text=True)
 def test_hexbin_empty():
     # From #3886: creating hexbin from empty dataset raises ValueError
-    ax = plt.gca()
+    fig, ax = plt.subplots()
     ax.hexbin([], [])
-
-
-@image_comparison(['hexbin_empty.png'], remove_text=True)
-def test_hexbin_log_empty():
+    fig, ax = plt.subplots()
     # From #23922: creating hexbin with log scaling from empty
     # dataset raises ValueError
-    ax = plt.gca()
     ax.hexbin([], [], bins='log')