bsim: Bluetooth: Mesh: extract common adv logic

Extracts logic related to waiting for packets from
`test_advertiser.c` and `test_beacon.c` to use in a sync mechanism.
Introduces a sync mechanism in bsim with the functions
`bt_mesh_test_send_over_adv` and `bt_mesh_test_wait_for_packet`

Extracts gatt-related logic from `test_advertiser.c` into a separate
file so it can be re-used.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>