Clean shutdown for testStaticMAC

Without waiting for mn to exit, OVS keeps listening on the
static listening port, which prevents UserSwitch from listening
on it (and starting up) in the next test.

mininet/test/test_walkthrough.py

@@ -253,6 +253,8 @@ class testWalkthrough( unittest.TestCase ):
             p.sendline( 'h%d ifconfig' % i )
             p.expect( 'HWaddr 00:00:00:00:00:0%d' % i )
             p.expect( self.prompt )
+        p.sendline( 'exit' )
+        p.expect( pexpect.EOF )
 
     def testSwitches( self ):
         "Run iperf test using user and ovsk switches"