session_track_system_variables_basic fails with ER_CANT_OPEN_LIBRARY

When there is no ha_example.so the test fails:

mysqltest: At line 134: query 'install soname 'ha_example.so'' failed: ER_CANT_OPEN_LIBRARY (1126): Can't open shared library '/home/midenok/src/mariadb/10.6/build/mysql-test/var/plugins/ha_example.so' (errno: 22, cannot open shared object file: No such file or directory)

MDEV-39207 have introduced install soname 'ha_example.so', which is wrong. The right way to load example plugin is to include have_example_plugin.inc. I do not see have_example_plugin.opt, so probably it does not load the library. Instead of direct install soname one should fix have_example_plugin to load the library if it is available or skip the test otherwise.