Make testsuite to pass

- Drop all tables in explain_json.test
- Tabular form should print ref='' when type='fulltext' (another peculiarity
  of the traditional EXPLAIN format)
- String_list::append_str should allocate memory for \0, too
- Some temporary code for EXPLAIN JSON and join buffering.

mysql-test/r/explain_json.result

@@ -174,5 +174,5 @@ EXPLAIN
     }
   }
 }
-drop table t1;
+drop table t1,t2;
 drop table t0;