tests: Fix wpas_mesh_open_vht20 false negative by using common finalizer

wpas_mesh_open_vht20 might fails with this message:

---------------
wlan0: Country code not reset back to 00: is US
wlan0: Country code cleared back to 00
---------------

This patch fixes the issue.

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>

tests/hwsim/test_wpas_mesh.py

@@ -1081,11 +1081,7 @@ def test_wpas_mesh_open_vht20(dev, apdev):
     try:
         _test_wpas_mesh_open_vht20(dev, apdev)
     finally:
-        dev[0].request("MESH_GROUP_REMOVE " + dev[0].ifname)
-        dev[1].request("MESH_GROUP_REMOVE " + dev[1].ifname)
-        subprocess.call(['iw', 'reg', 'set', '00'])
-        dev[0].flush_scan_cache()
-        dev[1].flush_scan_cache()
+        clear_reg_setting(dev)
 
 def _test_wpas_mesh_open_vht20(dev, apdev):
     check_mesh_support(dev[0])