tests: bluetooth: tester: Fix self-update test DFU/SR/FD/BV-59-C

DFU/SR/FD/BV-59-C tests Distributor self-update. Because the actuall
firmware swapping doesn't happen, we need to just simulate the swapping
by setting new Firmware ID sent by LT. We use slot to take firmware id
as by procedure, Fw Update Server doesn't know about it.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>