[dvs] Re-enable NAT test cases (#1205)

* Removed the xfail mark for NAT test cases
* Fixed the "test_NatInterfaceZone" test case failure

Signed-off-by: Akhilesh Samineni <akhilesh.samineni@broadcom.com>

tests/test_nat.py

@@ -7,9 +7,6 @@
 
 from swsscommon import swsscommon
 
-
-# FIXME: https://github.com/Azure/sonic-swss/issues/1199
-@pytest.mark.xfail(reason="DVS crashes during NAT test execution")
 class TestNat(object):
     def setup_db(self, dvs):
         self.appdb = swsscommon.DBConnector(0, dvs.redis_sock, 0)
@@ -84,8 +81,14 @@ def test_NatInterfaceZone(self, dvs, testlog):
         keys  = tbl.getKeys()
 
         (status, fvs) = tbl.get("Ethernet0")
-
-        assert fvs==(('NULL', 'NULL'), ('nat_zone', '1'))
+        assert status == True
+        assert len(fvs) > 0
+        zone = False
+        for fv in fvs:
+            if fv[0] == 'nat_zone' and fv[1] == '1':
+               zone = True
+               break
+        assert zone == True
 
 
     def test_AddNatStaticEntry(self, dvs, testlog):