tests: Fix bgp_ecmp_topo3 to look for a bit more state

tests: Fix bgp_ecmp_topo3 to look for a bit more state

During a local CI run, bgp_ecmp_topo3 was failing
to properly notice the fast-convergence command
issued before the interface is shut down.  As
such there exists a race condition where under
high load the zebra process can actually shut
an interface down before we have properly ensured
that fast convergence is on for ibgp.

Modify the test for in two ways:

1) Ensure that previous section makes sure
that we have properly converged for when we
bring back up the interfaces instead of
assuming that we have done so.

2) After issuing the fast-convergence command.
Ensure that bgp has fully processed it and is
ready to receive the interface down events
as triggers for shutting down the ibgp session.

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