selftests: forwarding: test for bridge sticky flag

This test adds an fdb entry with the sticky flag and sends traffic from
a different port with the same mac as a source address expecting the entry
to not change ports if the flag is operating correctly.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>