selftests: forwarding: lib: Add check_err_fail()

In the scale testing scenarios, one usually has a condition that is
expected to either fail, or pass, depending on which side of the scale
is being tested.

To capture this logic, add a function check_err_fail(), which dispatches
either to check_err() or check_fail(), depending on the value of the
first argument, should_fail.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>