this test loads sql_errlog plugin. then in a second connection
it triggers an error, this locks the plugin in that thd.
then the plugin is uninstalled in the default connection.
but that doesn't unload the plugin, as it's still locked. it'll
auto-unload after the foo connection is closed. without an explicit
disconnect it is closed after mysqltest exits and the post-test check
might still see sql_errlog not fully unoaded.