tools: ynl: don't install tests

tools: ynl: don't install tests

make's install target is meant for installing the production
artifacts, AFAIU. Don't install test_ynl_cli and test_ynl_ethtool
from under the main YNL install target. The install target
under tests/ is retained in case someone wants the tests
to be installed.

Fixes: 308b7dee3e5c ("tools: ynl: add YNL test framework")
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20260106163426.1468943-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>