After table->file->add_index() in mysql_alter_table() the table in the engine
has the intermediate temporary structure, it's neither the original nor the
final table structure (it'll be final after successful table->file->drop_index()
call). So, when add_index() fails with a unique key violation, we cannot simply
get the failed key number and easily map it to the key name and key structure via
table->key_info[key_no].

For now we'll create this "intermediate temporary structure", emulating InnoDB
internal rules.

This bug and the fix will go away in 10.0 that uses completely different online
alter table code.

mysql-test/t/alter_table_trans.test:
  mdev:5406