Fix intermittent issue on reboot in test_lldp_syncd (#15331)

At the start of test_lldp_syncd.py::test_lldp_entry_table_after_reboot
the test polls and checks that LLDP_ENTRY_TABLE keys match show lldp table output.

The eth0 port is added last so sometimes the entry keys will have it but the
lldp table output will not.  From debugging, this is the case since the
end of the previous test (I put a check on the keys vs lldp table output
and observed the missing eth0 at the end of
test_lldp_syncd.py::test_lldp_entry_table_after_lldp_restart)

Added a wait_until at the start of test_lldp_entry_table_after_reboot to
wait until the LLDP_ENTRY_TABLE keys match show lldp table output before
the tests starts to reboot.

tests/lldp/test_lldp_syncd.py

@@ -90,6 +90,13 @@ def get_show_lldp_table_output(duthost):
     return interface_list
 
 
+def check_lldp_table_keys(duthost, db_instance):
+    # Check if LLDP_ENTRY_TABLE keys match show lldp table output
+    lldp_entry_keys = get_lldp_entry_keys(db_instance)
+    show_lldp_table_int_list = get_show_lldp_table_output(duthost)
+    return sorted(lldp_entry_keys) == sorted(show_lldp_table_int_list)
+
+
 def assert_lldp_interfaces(
     lldp_entry_keys, show_lldp_table_int_list, lldpctl_interface
 ):
@@ -322,6 +329,12 @@ def test_lldp_entry_table_after_reboot(
     localhost, duthosts, enum_rand_one_per_hwsku_frontend_hostname, db_instance
 ):
     duthost = duthosts[enum_rand_one_per_hwsku_frontend_hostname]
+
+    # Verify LLDP_ENTRY_TABLE keys match show lldp table output at the start of test
+    keys_match = wait_until(30, 5, 0, check_lldp_table_keys, duthost, db_instance)
+    if not keys_match:
+        assert keys_match, "LLDP_ENTRY_TABLE keys do not match 'show lldp table' output"
+
     # reboot
     logging.info("Run cold reboot on DUT")
     reboot(