test-ovsdb: Fix memory leak reported by Valgrind.

Valgrind testcase 1967: simple idl, conditional, modify as delete due
to condition - C reports the following leak:
    json_array_create_empty (json.c:185)
    json_parser_push_array (json.c:1234)
    json_parser_input (json.c:1328)
    json_lex_input (json.c:945)
    json_parser_feed (json.c:1103)
    json_from_string (json.c:1025)
    parse_json (test-ovsdb.c:227)
    update_conditions (test-ovsdb.c:2324)
    do_idl (test-ovsdb.c:2389)
    ovs_cmdl_run_command (command-line.c:121)
    main (test-ovsdb.c:73)

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

tests/test-ovsdb.c

@@ -2344,6 +2344,7 @@ update_conditions(struct ovsdb_idl *idl, char *commands)
                 parse_link2_json_clause(idl, add_cmd, json->u.array.elems[i]);
             }
         }
+        json_destroy(json);
     }
 }