Add tests for stem3d.

These are somewhat like the original tests in matplotlib#6271, but consolidated
into a single image, and testing more properties.

Author: QuLogic

lib/mpl_toolkits/tests/baseline_images/test_mplot3d/stem3d.png

@@ -1203,6 +1203,35 @@ def test_errorbar3d():
     ax.legend()
 
 
+@image_comparison(['stem3d.png'], style='mpl20')
+def test_stem3d():
+    fig, axs = plt.subplots(2, 3, figsize=(8, 6),
+                            constrained_layout=True,
+                            subplot_kw={'projection': '3d'})
+
+    theta = np.linspace(0, 2*np.pi)
+    x = np.cos(theta - np.pi/2)
+    y = np.sin(theta - np.pi/2)
+    z = theta
+
+    for ax, zdir in zip(axs[0], ['x', 'y', 'z']):
+        ax.stem(x, y, z, orientation=zdir)
+        ax.set_title(f'orientation={zdir}')
+
+    x = np.linspace(-np.pi/2, np.pi/2, 20)
+    y = np.ones_like(x)
+    z = np.cos(x)
+
+    for ax, zdir in zip(axs[1], ['x', 'y', 'z']):
+        markerline, stemlines, baseline = ax.stem(
+            x, y, z,
+            linefmt='C4-.', markerfmt='C1D', basefmt='C2',
+            orientation=zdir)
+        ax.set_title(f'orientation={zdir}')
+        markerline.set(markerfacecolor='none', markeredgewidth=2)
+        baseline.set_linewidth(3)
+
+
 @image_comparison(["equal_box_aspect.png"], style="mpl20")
 def test_equal_box_aspect():
     from itertools import product, combinations