tests: Make dbus_scan_abort more robust

tests: Make dbus_scan_abort more robust

There was a race condition on the sequence where iface.AbortScan() is
immediately followed by iface.Scan(). If the driver event
(NL80211_CMD_SCAN_ABORTED) arrived after the following new scan request,
the D-Bus operation failed. This is not what this test case is trying to
check, so wait for an indication of the previous scan having terminated
properly before issuing the next scan.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>