tests: Bluetooth: Mesh: Test DFU Server update phase and img recovery

This adds BSim tests for DFU Server, where Firmware Distribution
procedure is stopped at given point. DFU Server is expected to save
phase and image index in persistent storage. Then new test is executed
that simulates reboot of device. Settings file is not cleared and after
setup device is expected to restore its DFU Server with correct phase
and idx.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>