Add config change validation to prevent bad bgp config state (sonic-net#19139)

What is the motivation for this PR?
bgp/test_bgp_suppress_fib.py::test_suppress_fib_stress failing

How did you do it?
Wait for config to properly take effect bgp/test_bgp_suppress_fib.py::test_credit_loop so subsequent tests can run on a clean state.

How did you verify/test it?
bgp/test_bgp_suppress_fib.py::test_suppress_fib_stress no longer has packet count mismatch failures.

Author: justin-wong-ce

tests/bgp/test_bgp_suppress_fib.py

@@ -1052,7 +1052,7 @@ def test_credit_loop(duthost, tbinfo, nbrhosts, ptfadapter, prepare_param, gener
     for exabgp_port, exabgp_port_v6, recv_port in zip(exabgp_port_list, exabgp_port_list_v6, recv_port_list):
         try:
             with allure.step("Disable bgp suppress-fib-pending function"):
-                config_bgp_suppress_fib(duthost, False)
+                config_bgp_suppress_fib(duthost, False, validate_result=True)
 
             with allure.step(
                     "Validate traffic is forwarded back to T2 VM and routes in HW table are removed by orchagent"):