topotests: isis-lfa add a switchover test after BFD down

topotests: isis-lfa add a switchover test after BFD down

Add a switchover test that consists in:
- Setting up ISIS BFD between rt1 and rt2
- The no link-detect setting on rt1 eth-rt2 is still present so that
  zebra does not take account linkdown events on this interface.
- Shutting down rt1 eth-rt2 from the switch side
- Wait for BFD to comes down

Check that the switchover between primary and backup happens before the
SPF re-computation.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>