tests: bfd_bgp_cbit_topo3 allow bgp to converge before testing

tests: bfd_bgp_cbit_topo3 allow bgp to converge before testing

This test was failing upstream a bunch of times.  Upon examining
the log files as well as the test script it was noticed that
the bfd peers were checked to see that they had come up.  But
both the timers used for bgp as well as not checking that bgp
has actually come up would cause the test to fail in subsuquent
steps if bgp has not come up.  Test that bgp peering is actually
established before testing link down events.  It's possible
this test might need to be revisited to ensure that the routes
are actually installed and ready to go before as well, but I am
not seeing that right now.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>