tests: TestCase.assertGreater is not available on Python 2.5

The jenkins build node has Python 2.5.X installed and the
assertGreater method is not available. Use assert_ until
we can use newer versions of Python.
diff --git a/openbsc/tests/vty_test_runner.py b/openbsc/tests/vty_test_runner.py
index fb01b89..778cde0 100644
--- a/openbsc/tests/vty_test_runner.py
+++ b/openbsc/tests/vty_test_runner.py
@@ -78,14 +78,14 @@
         # Enable periodic lu..
         self.vty.verify("periodic location update 60", [''])
         res = self.vty.command("write terminal")
-        self.assertGreater(res.find('periodic location update 60'), 0)
+        self.assert_(res.find('periodic location update 60') > 0)
         self.assertEquals(res.find('no periodic location update'), -1)
 
         # Now disable it..
         self.vty.verify("no periodic location update", [''])
         res = self.vty.command("write terminal")
         self.assertEquals(res.find('periodic location update 60'), -1)
-        self.assertGreater(res.find('no periodic location update'), 0)
+        self.assert_(res.find('no periodic location update') > 0)
 
 class TestVTYNAT(TestVTYBase):