Fix P4Runtime performance test

The test was failing intermittently in EOS because the ATE ingress
counters check for verifying the expected packetOut counters was done
right after injecting the traceroute/LLDP/GDP packets with no delay.

There is no guarantee that all the packetOut requests have been
guaranteed by that time. Added a 30s delay, to account for any trailing
inflight packets.

feature/experimental/p4rt/ate_tests/performance_test/performance_test.go

@@ -300,6 +300,7 @@ func testPktInPktOut(t *testing.T, args *testArgs) {
 
 		wg.Wait() // Wait for all four goroutines to finish before exiting.
 
+		time.Sleep(30 * time.Second)
 		// Check packet counters after packet out
 		counter1 := gnmi.Get(t, args.ate, gnmi.OC().Interface(port).Counters().InPkts().State())