bgp_router_test: Retry assertions of RIB for stability
bgp_router_test: Retry assertions of RIB for stability

To improve the stability of "bgp_router_test.py", this patch fixes to
retry assertions of RIB on a Quagga router after shutting down a
neighbor.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

j-kato/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,tamihiro/gobgp,tamihiro/gobgp,osrg/gobgp,osrg/gobgp,j-kato/gobgp,fujita/gobgp,osrg/gobgp,fujita/gobgp,a16/gobgp