lag_test: Add check to verify that an initial LACP packet was received (sonic-net#9343)

In the LacpTimingTest class, when getting the initial LACP packet, first
check to see that a packet was captured before trying to convert the
timestamp value of the tuple to a float. Otherwise, there'll be an error
about an invalid argument to float.

This doesn't necessarily prevent an error from being thrown, it just
makes it a more graceful/expected failure.

Signed-off-by: Saikrishna Arcot <[email protected]>

Author: saiarcot895

ansible/roles/test/files/acstests/lag_test.py

@@ -97,6 +97,7 @@ def getMedianInterval(self, masked_exp_pkt):
         # Verify two LACP packets.
         (rcv_device, rcv_port, rcv_pkt, last_pkt_time) = self.dataplane.poll(
             port_number=self.exp_iface, timeout=self.timeout, exp_pkt=masked_exp_pkt)
+        self.assertTrue(rcv_pkt is not None, "Failed to receive initial LACP packet\n")
         last_pkt_time = round(float(last_pkt_time), 2)
 
         for i in range(0, self.interval_count):