tests: Add some coverage for Device Discoverability exchange

tests: Add some coverage for Device Discoverability exchange

This is not really perfect, but something to get a bit more testing
coverage.. For proper discoverability mechanism validation, the P2P
client would need to go to sleep to avoid acknowledging the GO
Negotiation Request frame. Offchannel Listen mode operation on the P2P
Client with mac80211_hwsim is apparently not enough to avoid the
acknowledgement on the operating channel, so need to disconnect from the
group which removes the GO-to-P2P Client part of the discoverability
exchange in practice.

Signed-hostap: Jouni Malinen <j@w1.fi>