Most of the test cases are not interested in IPv6 Router Alerts,
so change the default behaviour of get_capture to filter out
these packets with the possibility of turning the filtering off,
for test cases which are interested in the RAs.
Change-Id: I0b5ee685f82c49cd32c6d6a4638eb3493d2988fc
Signed-off-by: Klement Sekera <ksekera@cisco.com>