Add missing type hints to tests. (#15027)

tests/test_test_utils.py

@@ -17,25 +17,25 @@ from tests.utils import MockClock
 
 
 class MockClockTestCase(unittest.TestCase):
-    def setUp(self):
+    def setUp(self) -> None:
         self.clock = MockClock()
 
-    def test_advance_time(self):
+    def test_advance_time(self) -> None:
         start_time = self.clock.time()
 
         self.clock.advance_time(20)
 
         self.assertEqual(20, self.clock.time() - start_time)
 
-    def test_later(self):
+    def test_later(self) -> None:
         invoked = [0, 0]
 
-        def _cb0():
+        def _cb0() -> None:
             invoked[0] = 1
 
         self.clock.call_later(10, _cb0)
 
-        def _cb1():
+        def _cb1() -> None:
             invoked[1] = 1
 
         self.clock.call_later(20, _cb1)
@@ -51,15 +51,15 @@ class MockClockTestCase(unittest.TestCase):
 
         self.assertTrue(invoked[1])
 
-    def test_cancel_later(self):
+    def test_cancel_later(self) -> None:
         invoked = [0, 0]
 
-        def _cb0():
+        def _cb0() -> None:
             invoked[0] = 1
 
         t0 = self.clock.call_later(10, _cb0)
 
-        def _cb1():
+        def _cb1() -> None:
             invoked[1] = 1
 
         self.clock.call_later(20, _cb1)