T6812: Fix smoketest iproute2 check (#4174)

In the new iproute2 package the link to `ip` was changed

$ file /usr/sbin/ip
/usr/sbin/ip: symbolic link to ../bin/ip

Fix smoketest

smoketest/scripts/system/test_iproute2.py

@@ -21,7 +21,7 @@ class TestIproute2(unittest.TestCase):
     def test_ip_is_symlink(self):
         # For an unknown reason VyOS 1.3.0-rc2 did not have a symlink from
         # /usr/sbin/ip -> /bin/ip - verify this now and forever
-        real_file = '/bin/ip'
+        real_file = '../bin/ip'
         symlink = '/usr/sbin/ip'
         self.assertTrue(os.path.islink(symlink))
         self.assertFalse(os.path.islink(real_file))