Tests and support for second FLVSS

Re #1409 and #1444:
* added support for two FLVSS
* added gtests for FLVSS

Bertrand, I did a little modification of your last commits https://github.com/opentx/opentx/compare/87a4f3526b...2d764e73fd 

I added support for two Sport lipo sensors. The code:
```c
 if (dataId != DATA_ID_FLVSS) {
   // TODO second sensor connected
}
```
needed some changes anyway. If left alone the second sensor would mess up the values from first sensor.

Now cells from second sensor are concatenated with the cells from first sensor. I believe this is the same as two sensors on hub do internally. For example if:
* sensor 1 has 3 cells
* sensor 2 has 2 cells
we get cellVolts[0] to cellVolts[4] populated.
Would you move it inside the hub structure?