tests: ensure BFD session is up before proceeding to the next step

tests: ensure BFD session is up before proceeding to the next step

In this topotest, BFD is configured at the end of step 13. However,
in certain cases where the testing machine is exceptionally fast (e.g.,
Donald's quantum computer), there is a possibility that the interface
shutdown event from step 14 may occur before BFD has had sufficient
time to establish the session, which leads to a test failure. To fix
this problem, ensure the BFD session is up before proceeding to the
next step.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>