tests: Bluetooth: Mesh: adapt bsim lpn tests for ext adv

mesh extended advertiser is not well tested. As the first step
the mesh babblesim tests should pass on extended advertiser as well
as on legacy advertiser. Minor fix to avoid lpn collisions in
friendship tests with multi lpns.

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