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>

tools/testing/selftests/net/forwarding/lib.sh

@@ -156,6 +156,19 @@ check_fail()
 	fi
 }
 
+check_err_fail()
+{
+	local should_fail=$1; shift
+	local err=$1; shift
+	local what=$1; shift
+
+	if ((should_fail)); then
+		check_fail $err "$what succeeded, but should have failed"
+	else
+		check_err $err "$what failed"
+	fi
+}
+
 log_test()
 {
 	local test_name=$1