selftests: net: dsa: Add a Makefile which installs the selftests

Add a Makefile which takes care of installing the selftests in
tools/testing/selftests/drivers/net/dsa. This can be used to install all
DSA specific selftests and forwarding.config using the same approach as
for the selftests in tools/testing/selftests/net/forwarding.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220727191642.480279-1-martin.blumenstingl@googlemail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>