Add voltage resolution unit test for MQ303A

examples/UnitTesting/UnitTesting.ino

@@ -246,6 +246,15 @@ test(MQ303A_Ethanol)
   assertEqual(PPM,ppmExp);
 }
 
+test(MQ303A_VoltResolution)
+{
+  float vRes = 4.7;
+  MQ303A.setVoltResolution(vRes);
+  MQ303A.setADC(100);            // provide dummy ADC value
+  MQ303A.readSensor(true);       // dummy read for MQ303A
+  assertEqualFloat(MQ303A.getVoltResolution(), vRes);
+}
+
 test(MQ309A_CO)
 {
   MQ309A.setRegressionMethod(1); //_PPM =  a*ratio^b