Add extra TrafficLight tests, and remove the redundant setattr function

- `CompositeDevice`s can't have any of their attributes set anyway

gpiozero/boards.py

@@ -710,13 +710,6 @@ class TrafficLights(LEDBoard):
             name = 'amber'
         return super(TrafficLights, self).__getattr__(name)
 
-    def __setattr__(self, name, value):
-        if name == 'amber' and self._display_yellow:
-            name = 'yellow'
-        elif name == 'yellow' and not self._display_yellow:
-            name = 'amber'
-        return super(TrafficLights, self).__setattr__(name, value)
-
 
 class PiTraffic(TrafficLights):
     """

tests/test_boards.py

@@ -555,6 +555,10 @@ def test_traffic_lights():
         assert red_pin.state
         assert not amber_pin.state
         assert not green_pin.state
+        board.amber.on()
+        assert amber_pin.state
+        board.yellow.off()
+        assert not amber_pin.state
     with TrafficLights(red=red_pin, yellow=amber_pin, green=green_pin) as board:
         board.yellow.on()
         assert not board.red.value
@@ -564,6 +568,8 @@ def test_traffic_lights():
         assert not red_pin.state
         assert amber_pin.state
         assert not green_pin.state
+        board.amber.off()
+        assert not amber_pin.state
 
 def test_traffic_lights_bad_init():
     with pytest.raises(ValueError):