Allow some diffs in h1 ps vs. h2 ps

Presumably daemons, race conditions, or ephemeral processes
can cause the ps output to vary slightly. We allow up to
two differing lines to account for this.

Fixes #651

mininet/test/test_walkthrough.py

@@ -126,10 +126,13 @@ class testWalkthrough( unittest.TestCase ):
         p.sendline( "s1 ps -a | egrep -v 'ps|grep'" )
         p.expect( self.prompt )
         s1Output = p.before
-        # strip command from ps output
-        h1Output = h1Output.split( '\n', 1 )[ 1 ]
-        s1Output = s1Output.split( '\n', 1 )[ 1 ]
-        self.assertEqual( h1Output, s1Output, 'h1 and s1 "ps" output differs')
+        # strip command from ps output and compute diffs
+        h1Output = h1Output.split( '\n' )[ 1: ]
+        s1Output = s1Output.split( '\n' )[ 1: ]
+        diffs = set( h1Output ).difference( set( s1Output ) )
+        # allow up to two diffs to account for daemons, etc.
+        self.assertTrue( len( diffs ) <= 2,
+                         'h1 and s1 "ps" output differ too much: %s' % diffs )
         p.sendline( 'exit' )
         p.wait()