Consider using a common parent directory under target

[dimo414]

Split from #2:

I was a little surprised testdir creates all its directories right in the target/ root; it ends up creating nine entries that seem like they could be nested under a common parent directory to keep the root dir cleaner. So in addition to / instead of using CARGO_TARGET_TMPDIR it might be nice to put all these paths under a single common directory such as target/testdir/dir-*.