selftests: mlxsw: Add target for mirror-to-gretap test on spectrum

Add a wrapper around mlxsw/mirror_gre_scale.sh that parameterized number
of offloadable mirrors on Spectrum machines.
Signed-off-by: NPetr Machata <petrm@mellanox.com>
Reviewed-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/drivers/net/mlxsw/spectrum/mirror_gre_scale.sh

+# SPDX-License-Identifier: GPL-2.0
+source ../mirror_gre_scale.sh
+
+mirror_gre_get_target()
+{
+	local should_fail=$1; shift
+
+	if ((! should_fail)); then
+		echo 3
+	else
+		echo 4
+	fi
+}