tester: fix fetching data of sensors
tester: fix fetching data of sensors

The given sensor ID was wrong because there might be holes in the
sequence 0..N as some of the sensors are controls.

Signed-off-by: Jan Viktorin <3eab6ac54c005c4408f608556a6663b7f2abede7@fit.vutbr.cz>

BeeeOn/server,BeeeOn/server,BeeeOn/server,BeeeOn/server