Use unique incrementing value for BTS LAC

Change-Id: I9f864bac05e39ec2fc305f774194799c3d8fe1b0
diff --git a/src/osmo_gsm_tester/resource.py b/src/osmo_gsm_tester/resource.py
index ecafa2f..9571131 100644
--- a/src/osmo_gsm_tester/resource.py
+++ b/src/osmo_gsm_tester/resource.py
@@ -210,6 +210,8 @@
     def next_msisdn(self, origin):
         return self.next_persistent_value('msisdn', '1000', schema.msisdn, util.msisdn_inc, origin)
 
+    def next_lac(self, origin):
+        return self.next_persistent_value('lac', '1', schema.uint16, lambda x: str((int(x)+1) % pow(2,16)), origin)
 
 class NoResourceExn(log.Error):
     pass