selftests: forwarding: fix learning_test when h1 supports IFF_UNICAST_FLT

The first host interface has by default no interest in receiving packets
MAC DA de:ad:be:ef:13:37, so it might drop them before they hit the tc
filter and this might confuse the selftest.

Enable promiscuous mode such that the filter properly counts received
packets.

Fixes: d4deb014

 ("selftests: forwarding: Add a test for FDB learning")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>