tools: ynl: don't install tests in /usr/bin/

tools: ynl: don't install tests in /usr/bin/

Until commit 790792ebc960 ("tools: ynl: don't install tests")
YNL selftests were installed with all the other YNL outputs.
That's no longer the case, as tests are not really production
artifacts. Let's not install them in /usr/bin at all, and
mirror kselftest format more closely:

For: make -C tools/net/ynl/tests/ install DESTDIR=tmp

tmp/usr/share/kselftest
              ├── ktap_helpers.sh
              └── ynl
                  ├── test_ynl_cli.sh
                  └── test_ynl_ethtool.sh

Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20260303163504.2084981-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>