make test: improve robustness and performance

Introduce an API which asserts empty capture for interface.
Throw exception in old API if the capture does not exist, thus
making it clear if the test expects packets to arrive or not.
Improve performance by not doing sleeps after starting the packet
generator, rather lazily deleting captures when needed.
Fix wrong usage of packet.show() in various tests.

Change-Id: I456cb23316eef99b3f35f80344fe595c4db9a21c
Signed-off-by: Klement Sekera <ksekera@cisco.com>

test/test_lb.py

@@ -1,7 +1,7 @@
 import socket
 
 from scapy.layers.inet import IP, UDP
-from scapy.layers.inet6 import IPv6
+from scapy.layers.inet6 import ICMPv6ND_RA, IPv6
 from scapy.layers.l2 import Ether, GRE
 from scapy.packet import Raw
 
@@ -95,10 +95,16 @@ class TestLB(VppTestCase):
         self.assertEqual(str(inner), str(self.info.data[IPver]))
 
     def checkCapture(self, gre4, isv4):
-        out = self.pg0.get_capture()
-        # This check is edited because RA appears in output, maybe disable RA?
-        # self.assertEqual(len(out), 0)
-        self.assertLess(len(out), 20)
+        # RA might appear in capture
+        try:
+            out = self.pg0.get_capture()
+            # filter out any IPv6 RAs from the capture
+            for p in out:
+                if (p.haslayer(ICMPv6ND_RA)):
+                    out.remove(p)
+            self.assertEqual(len(out), 0)
+        except:
+            pass
         out = self.pg1.get_capture()
         self.assertEqual(len(out), len(self.packets))