[GCU] Reload after lo test to recover default route (#16084)

What is the motivation for this PR?
default route test would fail after lo test

How did you do it?
Recover default route after test

How did you verify/test it?
E2E test

Author: wen587

tests/generic_config_updater/test_lo_interface.py

@@ -3,6 +3,8 @@
 import ipaddress
 
 from tests.common.helpers.assertions import pytest_assert
+from tests.common.utilities import wait_until
+from tests.common.config_reload import config_reload
 from tests.common.gu_utils import apply_patch, expect_op_success, expect_op_failure
 from tests.common.gu_utils import generate_tmpfile, delete_tmpfile
 from tests.common.gu_utils import format_json_patch_for_multiasic
@@ -73,6 +75,14 @@ def setup_env(duthosts, rand_one_dut_hostname, lo_intf):
         check_show_ip_intf(
             duthost, DEFAULT_LOOPBACK,
             [lo_intf["ipv6"].lower()], ["Vrf"], is_ipv4=False)
+
+        # Loopback interface removal will impact default route. Restart bgp to recover routes.
+        duthost.shell("sudo systemctl restart bgp")
+        if not wait_until(240, 10, 0, duthost.check_default_route):
+            logger.warning(
+                "Default routes not recovered after restart bgp, restoring with `config_reload`"
+            )
+            config_reload(duthost)
     finally:
         delete_checkpoint(duthost)