topotest: update the LDP VPLS test

topotest: update the LDP VPLS test

The final test case of this topotest wasn't really testing
anything. Do the following changes to fix this:
* Remove "no link-detect" from the zebra configs so that link down
  failures are detected immediately;
* After shutting down the interface between r1 and r2, wait five
  seconds before testing if the pseudowires reconverged through
  the r3 router.

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