tests: Make ap_mixed_security more robust

tests: Make ap_mixed_security more robust

This test case uses get_bss() with a BSSID to find a BSS entry. That can
result in failures if there are multiple BSS entries in wpa_supplicant
BSS table for the same BSSID, e.g., due to an earlier hidden SSID test
case. Explicitly clear the cfg80211 and wpa_supplicant scan caches at
the beginning of this test case to make it less likely for earlier test
cases to trigger a failure here.

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