Consider fleshing out `copy-tree` tests around symbolic links

[lread]

Issue

While working on #164, the question of what exactly copy-tree does with regards to symbolic links came up.

I think it would be helpful to understand the current behaviour with some tests.

And then, after that, we can decide if we want to make any changes.

Sound OK?

Initial Notes

Without understanding more, I found it a bit odd that copy-tree src and dest must be directories (and not symbolic links that point to directories).