tests: Bluetooth: Mesh: add proxy mix mesh bsim test

The commit adds proxy mix advertiser test that checks
mix of transmission between pb gatt, proxy and adv bearers.
The exact test scenario is described in proxy_mixin.sh file.

Additionally, the separate test configuration has been added.
All bsim mesh tests have been implemented not considering that
GATT frames are possible if PB-GATT or GATT Proxy are enabled.
The solution to avoid this misimplementation is to create
the separate configuration for tests with enabled
PB-GATT and GATT Proxy features. The commit adds this as well.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>
Signed-off-by: Artur Dobrynin <artur.dobrynin@nordicsemi.no>