tahoe cp -r: add tests for copying cyclic directories

[davidsarah]

Split from #104:

Tahoe directory structures can be cyclic; local ones can't. So there are two cases that need to be tested:

1. copying a cyclic Tahoe directory structure to another Tahoe directory (which should preserve the cycle structure)
2. copying a cyclic Tahoe directory structure to a local disk (which should probably cause an error).

It's not clear when case 2 should produce an error; it may be that it should be allowed to do some copying before detecting the cycle and then failing, in which case that should be documented.

If it doesn't currently do the right thing, that should be documented and tickets filed to fix it. See also #712.