iwlwifi: decouple testmode and iwl-test

The iwl-test flows were based on the cfg80211 testmode APIs.

To remove this coupling, the op mode (during the initialization
of the iwl_test object) is responsible to set the callbacks that
should be used by iwl-test to allocate skbs for events and replies
and to send events and replies.

The current op modes implement these callbacks based on the cfg80211
testmode APIs.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>