[rfc] add a TestFactory class or concept

[Alexander Belchenko]

Aaron Bentley пишет:
> MemoryTree is not close enough to a real working tree for me to be
> comfortable with it.  I don't trust it to do what I want, and I don't
> trust it to be a good enough emulation of a working tree.  I'd rather
> use a PreviewTree, since I know and trust the TreeTransform interface.

Aaron, I see you're always recommend to use PreviewTree and 
TreeTransform and it seems you think they both are the one right way for 
everyone.

But why you even don't comment on bug report about PreviewTree?
https://bugs.launchpad.net/bzr/+bug/251532