Bluetooth: Host: Add advertising events stress test

More information in the test description.

Why: At the time of writing, this test exposes a bug in the Bluetooth stack
where all the RX buffers are in use, and the controller driver fails to
synchronously allocate a command response buffer.

This is manifested in an assertion failure in `hci_common.c`.

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