tests: Fix scan result clearing in ssid_hidden*

tests: Fix scan result clearing in ssid_hidden*

These test cases were supposed to clear the cfg80211 and wpa_supplicant
scan caches in the end to avoid causing issues to the following test
cases. This did not work properly after introduction of the support for
aborting a pending scan. Fix this by using the flush_scan_cache()
function and waiting within the test case until the final scan operation
completes.

This issue was triggered by ssid_hidden/ssid_hidden2 followed by
ext_password_interworking (though, not every time).

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