selftests: forwarding: add TCPDUMP_EXTRA_FLAGS to lib.sh

For some use-cases we may want to change the tcpdump flags used in
tcpdump_start().  For instance, observing interfaces without the PROMISC
flag, e.g. to see what's really being forwarded to the bridge interface.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>