race condition in the test

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.

mysql-test/suite/sys_vars/r/sysvars_star.result

@@ -108,6 +108,7 @@ global_value_path	NULL
 connection default;
 select global_value_path from information_schema.system_variables where variable_name='plugin_maturity';
 global_value_path	var/my.cnf
+disconnect foo;
 drop user foo@localhost;
 set global low_priority_updates=default;
 uninstall soname 'sql_errlog';

mysql-test/suite/sys_vars/t/sysvars_star.test

@@ -34,8 +34,10 @@ select global_value_path from information_schema.system_variables where variable
 connection default;
 replace_regex /\/.*\//var\//;
 select global_value_path from information_schema.system_variables where variable_name='plugin_maturity';
-
+disconnect foo;
 drop user foo@localhost;
 set global low_priority_updates=default;
 disable_warnings;
 uninstall soname 'sql_errlog';
+let $count_sessions= 1;
+source include/wait_until_count_sessions.inc;