Clear subtree folder when re-running a test

[thomasjm]

Otherwise the run might be affected by the non-empty contents.

(Or, make a new numbered folder for each new run?)

[thomasjm]

Really the problem also applies not just to the "it" node, but to any ancestor node that puts things in its folder. It seems a little surprising to clear ancestor folders, since that could blow away results pertaining to other tests.

So, make a new numbered folder for every ancestor node? Or, just forget about this and count on tests to do idempotent things? The former would be most "correct".