selftests: forwarding: Fix layer 2 miss test syntax

The test currently specifies "l2_miss" as "true" / "false", but the
version that eventually landed in iproute2 uses "1" / "0" [1]. Align the
test accordingly.

[1] https://lore.kernel.org/netdev/20230607153550.3829340-1-idosch@nvidia.com/



Fixes: 8c33266a ("selftests: forwarding: Add layer 2 miss test cases")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>