Add a test hook for the PX4IO interface. Wire up some simple tests for the serial interface.

Signal quality looks good at 1.5Mbps. Transmit timing is ~450µs per packet, ~20µs packet-to-packet delay spinning in a loop transmitting.

src/drivers/px4io/interface_i2c.cpp

@@ -69,6 +69,8 @@ public:
 
 	virtual bool	ok();
 
+	virtual int	test(unsigned mode);
+
 private:
 	static const unsigned	_retries = 2;
 
@@ -107,6 +109,12 @@ PX4IO_I2C::ok()
 	return true;
 }
 
+int
+PX4IO_I2C::test(unsigned mode)
+{
+	return 0;
+}
+
 int
 PX4IO_I2C::set_reg(uint8_t page, uint8_t offset, const uint16_t *values, unsigned num_values)
 {