Added smoke test for Axes.matshow to test_datetime.py (matplotlib#27474)

Author: ConstableCatnip

lib/matplotlib/tests/test_datetime.py

@@ -542,11 +542,16 @@ def test_loglog(self):
         fig, ax = plt.subplots()
         ax.loglog(...)
 
-    @pytest.mark.xfail(reason="Test for matshow not written yet")
     @mpl.style.context("default")
     def test_matshow(self):
+        a = np.diag(range(5))
+        dt_start = datetime.datetime(1980, 4, 15)
+        dt_end = datetime.datetime(2020, 11, 11)
+        extent = (dt_start, dt_end, dt_start, dt_end)
         fig, ax = plt.subplots()
-        ax.matshow(...)
+        ax.matshow(a, extent=extent)
+        for label in ax.get_xticklabels():
+            label.set_rotation(90)
 
     @pytest.mark.xfail(reason="Test for pcolor not written yet")
     @mpl.style.context("default")