Update test_diagram.py

Update unit test to reflect the file name change.

tests/test_diagram.py

@@ -101,11 +101,11 @@ class DiagramTest(unittest.TestCase):
         self.assertTrue(os.path.exists(f"{self.name}.png"))
 
     def test_empty_name(self):
-      """Check that providing an empty name don't crash, but save in a diagrams_file.xxx file."""
-      self.name = 'diagrams_file'
+      """Check that providing an empty name don't crash, but save in a diagrams_image.xxx file."""
+      self.name = 'diagrams_image'
       with Diagram(show=False):
           Node("node1")
-      self.assertTrue(os.path.exists(f"diagrams_file.png"))
+      self.assertTrue(os.path.exists(f"{self.name}.png"))
 
 
 class ClusterTest(unittest.TestCase):