px4io: add support for test_motor

And fix some IO bugs:
- PX4IO_P_STATUS_FLAGS_RAW_PWM was never cleared. This meant that after a
  'pwm test' command, normal mixing was not possible anymore.
  Fixed by remembering when we are in test mode and not sending
  PX4IO_PAGE_CONTROLS during that time. PX4IO_P_STATUS_FLAGS_RAW_PWM is
  cleared when PX4IO_PAGE_CONTROLS are received.
- when entering test mode w/o specifying all channels, the other channels
  were not set at all, which means they could still be set to values from
  a previous test call.
  This is fixed by setting all channels to disarmed when entering/leaving
  test mode.

src/modules/px4iofirmware/registers.c

@@ -291,6 +291,8 @@ registers_set(uint8_t page, uint8_t offset, const uint16_t *values, unsigned num
 			values++;
 		}
 
+		r_status_flags &= ~PX4IO_P_STATUS_FLAGS_RAW_PWM;
+
 		system_state.fmu_data_received_time = hrt_absolute_time();
 
 		break;