selftests: forwarding: fix error message in learning_test

When packets are not received, they aren't received on $host1_if, so the
message talking about the second host not receiving them is incorrect.
Fix it.

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>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>