[sonic-cfggen]: Add bgp asn for yang validation (sonic-net#9640)

Why I did it
Config db schema generated by minigraph can’t pass yang validation, bgp_asn must not be None.

How I did it
Update sampe-voq-graph.xml to add bgp_asn.

How to verify it
Build sonic-config-engine.
Run command 'sonic-cfggen -m tests/sample-voq-graph.xml -p tests/voq-sample-port-config.ini --print-data', and check bgp_asn.

Signed-off-by: Gang Lv ganglv@microsoft.com

src/sonic-config-engine/tests/sample-voq-graph.xml

@@ -1,7 +1,14 @@
 <DeviceMiniGraph xmlns="Microsoft.Search.Autopilot.Evolution" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
   <CpgDec>
     <PeeringSessions/>
-    <Routers xmlns:a="http://schemas.datacontract.org/2004/07/Microsoft.Search.Autopilot.Evolution"/>
+    <Routers xmlns:a="http://schemas.datacontract.org/2004/07/Microsoft.Search.Autopilot.Evolution">
+      <a:BGPRouterDeclaration>
+        <a:ASN>65100</a:ASN>
+          <a:Hostname>linecard-1</a:Hostname>
+          <a:Peers/>
+        <a:RouteMaps/>
+      </a:BGPRouterDeclaration>
+    </Routers>
   </CpgDec>
   <DpgDec>
     <DeviceDataPlaneInfo>