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/t/explain_json.test

@@ -38,5 +38,5 @@ explain format=json select * from t2 where a1=1 or (b1=2 and b2=3);
 
 explain format=json select * from t0,t2 where t2.b1=t0.a and t2.b2=4;
 
-drop table t1;
+drop table t1,t2;
 drop table t0;