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.

tester/i2c.go

@@ -1,5 +1,7 @@
 package tester
 
+import "fmt"
+
 // I2CBus implements the I2C interface in memory for testing.
 type I2CBus struct {
 	c       Failer
@@ -16,10 +18,24 @@ func NewI2CBus(c Failer) *I2CBus {
 }
 
 // AddDevice adds a new mock device to the mock I2C bus.
+// It panics if a device with the same address is added more than once.
 func (bus *I2CBus) AddDevice(d *I2CDevice) {
+	for _, dev := range bus.devices {
+		if dev.Addr() == d.addr {
+			panic(fmt.Errorf("device already added at address %#x", d))
+		}
+	}
 	bus.devices = append(bus.devices, d)
 }
 
+// NewDevice creates a new device with the given address
+// and adds it to the mock I2C bus.
+func (bus *I2CBus) NewDevice(addr uint8) *I2CDevice {
+	dev := NewI2CDevice(bus.c, addr)
+	bus.AddDevice(dev)
+	return dev
+}
+
 // ReadRegister implements I2C.ReadRegister.
 func (bus *I2CBus) ReadRegister(addr uint8, r uint8, buf []byte) error {
 	return bus.FindDevice(addr).ReadRegister(r, buf)