tests: bluetooth: tester: Fix BLOB server model pointer

PTS uses BLOB server model instantiated by DFD server for MBT tests as
it is the first BLOB server model in the device composition data. DFU
server, which also has BLOB server model, is instantiated after DFD
server.

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