Add test for a proprietary HCI Vendor-Specific event.
In the test the controller is replaced with a small stub controller,
with limited support of the initialization flow from the host.
After initialization, a HCI prop event callback is registered and an
prop event is triggered and sent to the host. It is then verified that
the registered callback is called.
Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>