Try to prime the pump to avoid PACKET_INs during iperf test

Background: the reference controller is reactive and installs exact
match rules. By attempting to start a telnet session we make sure that
the ARP caches and TCP flow rules are set up (in one direction at
least) before the test starts.

This is intended to help with #413

examples/linearbandwidth.py

@@ -93,6 +93,9 @@ def linearBandwidthTest( lengths ):
         print "*** testing bandwidth"
         for n in lengths:
             src, dst = net.hosts[ 0 ], net.hosts[ n ]
+            # Try to prime the pump to reduce PACKET_INs during test
+            # since the reference controller is reactive
+            src.cmd( 'telnet', dst.IP(), '5001' )
             print "testing", src.name, "<->", dst.name,
             bandwidth = net.iperf( [ src, dst ] )
             print bandwidth