[ipfwd/test_nhop_group]: Support multi-asic in interface flap test

* Apply arp eviction to specific asic
* Add test to multi asic pr test scripts

Signed-off-by: Liam Kearney <liamkearney@microsoft.com>

.azure-pipelines/pr_test_scripts.yaml

@@ -449,6 +449,7 @@ multi-asic-t1-lag:
   - http/test_http_copy.py
   - telemetry/test_telemetry_cert_rotation.py
   - telemetry/test_telemetry.py
+  - ipfwd/test_nhop_group.py
 
 dpu:
   - dash/test_dash_vnet.py

tests/ipfwd/test_nhop_group.py

@@ -836,7 +836,7 @@ def test_nhop_group_interface_flap(duthosts, enum_rand_one_per_hwsku_frontend_ho
 
         # Enable kernel flag to not evict ARP entries when the interface goes down
         # and shut the fanout switch ports.
-        duthost.shell(arp_noevict_cmd % gather_facts['src_router_intf_name'])
+        asic.command(arp_noevict_cmd % gather_facts['src_router_intf_name'])
         for i in range(0, len(gather_facts['src_port'])):
             fanout, fanout_port = fanout_switch_port_lookup(fanouthosts, duthost.hostname,
                                                             gather_facts['src_port'][i])
@@ -876,6 +876,6 @@ def test_nhop_group_interface_flap(duthosts, enum_rand_one_per_hwsku_frontend_ho
         logger.info("portstats: %s", result['stdout'])
 
     finally:
-        duthost.shell(arp_evict_cmd % gather_facts['src_router_intf_name'])
+        asic.command(arp_evict_cmd % gather_facts['src_router_intf_name'])
         nhop.delete_routes()
         arplist.clean_up()