Consider alternative to in_temporary_directory

[casey]

I think that in_temporary_directory has some downsides:

• It requires that tests not run in parallel. This isn't a problem now, but if the tests got more numerous or slower, it could be come one.
• Tests that want to get or set the current directory need to use in_temporary_directory. If other tests didn't modify in_temporary_directory, then only those tests that needed to set the current dir would need to use something, which I think is the default expectation.

One nice thing about in_temporary_directory is that cmd! invocations are as close as possible to what users would be doing, and no extra mechanisms are in-use which might cause behavior to be different, or to cause unrelated tests to fail.