tests: More robust connect command testing

Avoid an invalid failure case due to scan results being left behind from
connect_cmd_bssid_hint when executing connect_cmd_reject_assoc by
explicitly clearing the scan results from dev5. This fixes an error case
that happened with the following test case sequence:
connect_cmd_bssid_hint connect_cmd_reject_assoc

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