Fix real pin tests ... and some other bits

The real pin tests were broken by the new factory stuff. This commit
fixes them up, and fixes up a few other bits besides (like why the
pigpio PWM tests were failing, why RPi.GPIO sometimes segfaulted on PWM
tests, etc.)

It also causes the real pin tests to run against MockPin (thanks to
@lurch for the suggestion!). This required some tweaks to MockPin to
make it emulate physically pulled up pins itself (which in turn
necessitated changing quite a few pin numbers in the main test suite
because we were using 2 and 3 everywhere), and to allow one MockPin to
drive another. Anyway, everything's working now including all the tests
on a Pi (haven't tried RPIO yet, but only because I'm on a Pi3 -
everything else works with overall coverage of 88% :).

tests/test_devices.py

@@ -65,7 +65,7 @@ def test_device_reopen_same_pin():
         assert device.pin is None
 
 def test_device_repr():
-    with GPIODevice(2) as device:
+    with GPIODevice(4) as device:
         assert repr(device) == '<gpiozero.GPIODevice object on pin %s, is_active=False>' % device.pin
 
 def test_device_repr_after_close():
@@ -85,18 +85,18 @@ def test_device_context_manager():
 
 def test_composite_device_sequence():
     with CompositeDevice(
-            InputDevice(2),
-            InputDevice(3)
+            InputDevice(4),
+            InputDevice(5)
             ) as device:
         assert len(device) == 2
-        assert device[0].pin.number == 2
-        assert device[1].pin.number == 3
+        assert device[0].pin.number == 4
+        assert device[1].pin.number == 5
         assert device.namedtuple._fields == ('_0', '_1')
 
 def test_composite_device_values():
     with CompositeDevice(
-            InputDevice(2),
-            InputDevice(3)
+            InputDevice(4),
+            InputDevice(5)
             ) as device:
         assert device.value == (0, 0)
         assert not device.is_active
@@ -106,8 +106,8 @@ def test_composite_device_values():
 
 def test_composite_device_named():
     with CompositeDevice(
-            foo=InputDevice(2),
-            bar=InputDevice(3),
+            foo=InputDevice(4),
+            bar=InputDevice(5),
             _order=('foo', 'bar')
             ) as device:
         assert device.namedtuple._fields == ('foo', 'bar')
@@ -126,8 +126,8 @@ def test_composite_device_bad_init():
 
 def test_composite_device_read_only():
     with CompositeDevice(
-        foo=InputDevice(2),
-        bar=InputDevice(3)
+        foo=InputDevice(4),
+        bar=InputDevice(5)
         ) as device:
         with pytest.raises(AttributeError):
             device.foo = 1