Fix test case: generic_config_updater/test_ipv6.py::test_ipv6_neighbo…

…r_admin_change

This case fails every time on our community setup because it tries to replace the value of config entry " BGP_NEIGHBOR/fc00::12/admin_status", while this entry doesn't exist in the default running configuratuion.(The admin_status is up by default, but it doesn't show in the configuration.)
To fix this,  need to add this config before replace it.
And add a topology mark for the test according to the description in sonic-net#5061 .

tests/generic_config_updater/test_ipv6.py

@@ -10,6 +10,9 @@
 
 logger = logging.getLogger(__name__)
 
+pytestmark = [
+ pytest.mark.topology('t0', 't1'),
+]
 
 @pytest.fixture(autouse=True)
 def ensure_dut_readiness(duthost):
@@ -153,6 +156,11 @@ def test_invalid_ipv6_neighbor(duthost, ensure_dut_readiness, op, dummy_neighbor
 def test_ipv6_neighbor_admin_change(duthost):
  ipv6_neighbor_address, ipv6_neighbor_config = get_ipv6_neighbor(duthost)
  json_patch = [
+ {
+ "op": "add",
+ "path": "/BGP_NEIGHBOR/{}/admin_status".format(ipv6_neighbor_address),
+ "value": "up"
+ },
  {
  "op": "replace",
  "path": "/BGP_NEIGHBOR/{}/admin_status".format(ipv6_neighbor_address),