tests: Bluetooth: add babblesim connection stress test

This is supposed to stress the host buffer and metadata handling.

It is not really testing any particular thing, ie. the pass criteria is
just that each peripheral can send and receive a fixed number of GATT
notifications without any device crashing or locking up.

Original version by Ahmed Moheib (from cloud2ground).

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>