tests: Bluetooth: Mesh: run bsim mesh tests from non-preemtible thread

BLE Mesh stack has been designed to work in non-preemtible
environment. The PR fixes running bsim mesh test from main thread
that is preemptible. Running mesh tests causes reentering in the
same mesh functionality twice and rewriting ongoing data.

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