tests: Addition of AutoRP Discovery uncovered broken PIM test

tests: Addition of AutoRP Discovery uncovered broken PIM test

With AutoRP discovery running by default, that adds a new
IGMP group that needs to be accounted for in IGMP output.

For pim.py
  The clear IGMP interfaces function is in a broken state. It was
  already ignoring any errors and returned true always, but with
  the addition of the AutoRP discovery group, you could end up
  with a different group order in the json which would cause a key
  error making the test fail. For now I just added a check to
  avoid the key error.

Signed-off-by: Nathan Bahr <nbahr@atcorp.com>