AP_InertialSensor: fixed start timer for temperature in SITL

when we are doing an autotest we want the temperature to start
climbing when we first set SIM_IMUT_FIXED=0

libraries/AP_InertialSensor/AP_InertialSensor_SITL.cpp

@@ -50,8 +50,12 @@ float AP_InertialSensor_SITL::get_temperature(void)
         // user wants fixed temperature
         return sitl->imu_temp_fixed;
     }
+    uint32_t now = AP_HAL::millis();
+    if (temp_start_ms == 0) {
+        temp_start_ms = now;
+    }
     // follow a curve with given start, end and time constant
-    const float tsec = AP_HAL::millis() * 0.001f;
+    const float tsec = (AP_HAL::millis() - temp_start_ms) * 0.001f;
     const float T0 = sitl->imu_temp_start;
     const float T1 = sitl->imu_temp_end;
     const float tconst = sitl->imu_temp_tconst;

libraries/AP_InertialSensor/AP_InertialSensor_SITL.h

@@ -43,6 +43,7 @@ private:
     float accel_time;
     float gyro_motor_phase[12];
     float accel_motor_phase[12];
+    uint32_t temp_start_ms;
 
     static uint8_t bus_id;
 };