Steve, thanks for the suggestions.  

Looking at the other tests, `test_prompt` was the only one (prior to the original change) that didn't create a venv, so maybe actually creating the venv instead of just instantiating the class were testing that everything worked together?  For example, see `test_isolation` and `test_symlinking`.  

I've added the code to explicitly remove the directory and I also slightly reorganized the create.  Not all the tests use the `run_with_capture`, but at the same time, none of the ones that use it do anything with the output, so I thought it wouldn't hurt to add it.

I tried to submit the tests just to the buildbots based on the devguide, but they were failing for other reasons, so I made the PR.  I don't think we'll know if this fixes the bots until the PR is merged, but if one of you could take a look and approve it, I'd appreciate it.  And then I'll keep my fingers crossed.   :-)