tests: Ensure packets have a chance to arrive in test_multicast_pim_sm_topo4.py

tests: Ensure packets have a chance to arrive in test_multicast_pim_sm_topo4.py

The test is doing this:

a) gather interface data about packets sent
b) shut interface
c) no shut interface
d) gather interface data about packets sent
e) compare a to d and fail if packets sent/received has not incremented

The problem is, of course, that under heavy system load insufficient time
might not have passed for packets to be sent between c and d.  Add up to
35 seconds of looking for packet data being incremented else heavily
loaded systems may never show that data is being sent.

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