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)
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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

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1 file changed
tree: 14b6cfccb3b714498b93c6b3538bceb5b7389dca
  1. contrib/
  2. debian/
  3. openbsc/
  4. .gitignore