tests: one more ipfix flush cli to api replacement

Type: fix
Fixes: 21b83e96d6076cda01ee80343840bc1166557150

Change-Id: I8216127ddbbf3301d2602bab54dbc433233387a2
Signed-off-by: Vratko Polak <vrpolak@cisco.com>
diff --git a/test/test_nat.py b/test/test_nat.py
index ccc9be7..a678621 100644
--- a/test/test_nat.py
+++ b/test/test_nat.py
@@ -2863,7 +2863,7 @@
         self.pg_start()
         self.pg1.assert_nothing_captured()
         sleep(1)
-        self.vapi.cli("ipfix flush")
+        self.vapi.ipfix_flush()
         capture = self.pg3.get_capture(9)
         ipfix = IPFIXDecoder()
         # first load template