mcp23017: use new tester package

Also change the `tester` package slightly to use an exposed `Registers`
array rather adding yet another accessor method to retrieve a register
value. This seems to me more transparent and "obvious" - we aren't trying
to hide the fact that there's just a simple memory store there.
Also unexport the `assertRegisterRange` method which was never intended
to be part of the public API.

lis2mdl/lis2mdl_test.go

@@ -18,13 +18,13 @@ func TestWhoAmI(t *testing.T) {
 	c := qt.New(t)
 	bus := tester.NewI2CBus(c)
 	fake := tester.NewI2CDevice(c, ADDRESS)
-	fake.SetupRegisters(defaultRegisters())
+	copy(fake.Registers[:], defaultRegisters())
 	bus.AddDevice(fake)
 
 	dev := New(bus)
 	c.Assert(dev.Connected(), qt.Equals, true)
 
-	fake.SetupRegister(WHO_AM_I, 0x99)
+	fake.Registers[WHO_AM_I] = 0x99
 	c.Assert(dev.Connected(), qt.Equals, false)
 }