test: Return None for success in test_timezone_env() (#671)

- test_timezone_env() is producing the warning,
  "PytestReturnNotNoneWarning: Test functions should return None, but
  src/cmdeploy/tests/online/test_1_basic.py::test_timezone_env
  returned <class 'bool'>".
- Revised test_timezone_env() to return None for success instead of
  True.

cmdeploy/src/cmdeploy/tests/online/test_1_basic.py

@@ -69,7 +69,7 @@ def test_timezone_env(remote):
     for line in remote.iter_output("env"):
         print(line)
         if line == "tz=:/etc/localtime":
-            return True
+            return
     pytest.fail("TZ is not set")