Add onewire smoke test (#104)

* set SPI speed and mode in spi smoketest
* add one-wire smoke test

experimental/devices/ds248x/dev.go

@@ -37,6 +37,13 @@ func (d *Dev) String() string {
 	return fmt.Sprintf("ds248x")
 }
 
+// Close drops the I²C bus handle and sets a persistent error.
+func (d *Dev) Close() error {
+	d.i2c = nil
+	d.err = fmt.Errorf("ds248x: invalid operation on closed bus")
+	return nil
+}
+
 // Tx performs a bus transaction, sending and receiving bytes, and
 // ending by pulling the bus high either weakly or strongly depending
 // on the value of power.

experimental/devices/ds248x/ds248x.go

@@ -42,7 +42,11 @@ type Opts struct {
 	PullupRes      PupOhm        // passive pull-up resistance, true: 500Ω, false: 1kΩ
 }
 
-// New returns an device object that communicates over I²C to the DS2482/DS2483 controller.
+// New returns a device object that communicates over I²C to the DS2482/DS2483
+// controller.
+//
+// This device object implements onewire.Bus and can be used to
+// access devices on the bus.
 func New(i i2c.Bus, opts *Opts) (*Dev, error) {
 	addr := uint16(0x18)
 	if opts != nil {