Skip mysql_json_mysql_upgrade if plugin is not built

Do not use --source directive as we don't want to load the plugin when
the test starts.

mysql-test/main/mysql_json_mysql_upgrade.test

@@ -6,6 +6,10 @@
 -- source include/have_utf8.inc
 -- source include/mysql_upgrade_preparation.inc
 
+if (!$TYPE_MYSQL_JSON_SO) {
+  skip Need MYSQL_JSON plugin;
+}
+
 SET NAMES utf8;
 
 let $MYSQLD_DATADIR= `select @@datadir`;