tests: Bluetooth: Mesh: fix reprovisioning test with ext adv

Extended advertiser has better latency without any delays than legacy one.
If couple of config model commands start sequently then
the first ones are received normally but the next one are transmitted in
parallel with leftover communication (retransmissions) of the previous one.
That causes collisions and broken test despite the functionality itself
is ok. Adding delays between commands solves crossovering tx time.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>