tests: Make dbus_old_wps_pbc more robust
Flush the cfg80211 scan cache explicitly to avoid false failure reports
if a BSS entry from an earlier test case remain. Such a failure could be
hit, e.g., with the following test case sequence:
wpas_mesh_mode_scan p2p_channel_random_social dbus_old_wps_pbc
Signed-off-by: Jouni Malinen <j@w1.fi>