tests: Flush scan results before checking alloc failure

tests: Flush scan results before checking alloc failure

When run after other tests, It was likely that the target
bss was already present in scan_fail, so the
scan_for_bss() wouldn't trip the allocation failure in
wpa_bss_add(). Flush the scan results before the scan to
ensure wpa_bss_add() is called and consistently pass
scan_fail.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>