fix(tests): set lower for case sensitive system (#805)

tests/test_c4.py

@@ -10,7 +10,7 @@ from diagrams.c4 import Person, Container, Database, System, SystemBoundary, Rel
 
 class C4Test(unittest.TestCase):
     def setUp(self):
-        self.name = "diagram-" + "".join([random.choice(string.hexdigits) for n in range(7)])
+        self.name = "diagram-" + "".join([random.choice(string.hexdigits) for n in range(7)]).lower()
 
     def tearDown(self):
         setdiagram(None)