tests: bfd_isis_topo1 expects unreasonable convergence times under load

tests: bfd_isis_topo1 expects unreasonable convergence times under load

When our ci test system is under high load, expecting bfd to converge
in under 2 seconds is not going to happen.  Modify the test suites
to just ensure that things converge.  If we need actual functional
testing of bfd response times the topotests are not an appropriate place
to do this or we need to modify the test system to gather the data for
how long it takes after the tests are run.

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