tests: add topotest for PG remote-as add del

tests: add topotest for PG remote-as add del

This topotest covers the PR-15574's coverage
where a change is not to delete neighbors when
the associated peer-group's remote-as is removed.

Ticket: #3828243
Testing:

test_bgp_peer-group.py::test_bgp_peer_group

---------------------------------------------------------------------------- live log call -----------------------------------------------------------------------------
2024-03-29 18:12:22,608 INFO: r1: checking if daemons are running
2024-03-29 18:12:22,802 INFO: r2: checking if daemons are running
2024-03-29 18:12:22,911 INFO: r3: checking if daemons are running
2024-03-29 18:12:23,015 INFO: topo: Remove bgp peer-group PG1 remote-as neighbor should be retained
2024-03-29 18:12:25,605 INFO: topo: Re-add bgp peer-group PG1 remote-as neighbor should be established

----------------------------------------------------------- generated xml file: /tmp/topotests/topotests.xml -----------------------------------------------------------
========================================================================== 2 passed in 17.63s ==========================================================================

Signed-off-by: Chirag Shah <chirag@nvidia.com>