AP_TemperatureSensor: allow testing of temperature sending in SITL

libraries/AP_TemperatureSensor/TSYS01.h

@@ -16,12 +16,21 @@ public:
 
     bool init(void);
     float temperature(void) { return _temperature; } // temperature in degrees C
-    bool healthy(void) { return _healthy; } // do we have a valid temperature reading?
+    bool healthy(void) { // do we have a valid temperature reading?
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        return true;
+#endif
+        return _healthy;
+    }
 
     AP_HAL::OwnPtr<AP_HAL::Device> _dev;
 
 private:
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+    float _temperature = 42.42; // degrees C
+#else
     float _temperature; // degrees C
+#endif
     bool _healthy; // we have a valid temperature reading to report
     uint16_t _k[5]; // internal calibration for temperature calculation
     bool _reset(void); // reset device