nat/vty: Use different port for the mock MSC

Update the comment to reflect that the NAT itself will bind to port
5000 and then the mock MSC will fail to bind to it. Try to move the
mock MSC to another port.

Could fix:

======================================================================
ERROR: testBSCreload (__main__.TestVTYNAT)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./vty_test_runner.py", line 654, in testBSCreload
    msc = nat_msc_test(self, ip)
  File "./vty_test_runner.py", line 1101, in nat_msc_test
    msc.bind((ip, 5000))
  File "<string>", line 1, in bind
error: [Errno 98] Address already in use

----------------------------------------------------------------------

openbsc/tests/vty_test_runner.py

@@ -627,7 +627,10 @@ class TestVTYNAT(TestVTYGenericBSC):
         return (4244, "src/osmo-bsc_nat/osmo-bsc_nat",  "OsmoBSCNAT", "nat")
 
     def testBSCreload(self):
+        # Use different port for the mock msc to avoid clashing with
+        # the osmo-bsc_nat itself
         ip = "127.0.0.1"
+        port = 5001
         self.vty.enable()
         bscs1 = self.vty.command("show bscs-config")
         nat_bsc_reload(self)
@@ -650,8 +653,8 @@ class TestVTYNAT(TestVTYGenericBSC):
         self.vty.command("token xyu")
         self.vty.command("end")
 
-        nat_msc_ip(self, ip)
-        msc = nat_msc_test(self, ip)
+        nat_msc_ip(self, ip, port)
+        msc = nat_msc_test(self, ip, port)
         b0 = nat_bsc_sock_test(0, "lol")
         b1 = nat_bsc_sock_test(1, "xyu")
         b2 = nat_bsc_sock_test(5, "key")
@@ -1086,19 +1089,20 @@ def nat_bsc_reload(x):
     x.vty.command("bscs-config-file bscs.config")
     x.vty.command("end")
 
-def nat_msc_ip(x, ip):
+def nat_msc_ip(x, ip, port):
     x.vty.command("configure terminal")
     x.vty.command("nat")
     x.vty.command("msc ip " + ip)
+    x.vty.command("msc port " + port)
     x.vty.command("end")
 
 def data2str(d):
     return "".join("{:02x}".format(ord(c)) for c in d)
 
-def nat_msc_test(x, ip, verbose = False):
+def nat_msc_test(x, ip, port, verbose = False):
     msc = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
     msc.settimeout(32)
-    msc.bind((ip, 5000))
+    msc.bind((ip, port))
     msc.listen(5)
     if (verbose):
         print "MSC is ready at " + ip