The patch looks good, I'd just move _test_cwd inside the function and drop the [:-3] from TESTFN, but apart from that it's OK. I also agree that functools.wraps should be added.

To summarize the discussion we had on #python-dev:
1) the context manager should always create a writable cwd and to be able to run with -J it should contain the process id. Using TESTFN as first-level dir solves both the issues;
2) a suffix is added to TESTFN to let the tests use TESTFN as a valid filename;
3) the second-level dir is 'tempcwd' by default or can be passed to the function in case a test needs a specific name for the cwd;

The result will be something like '@test_xxxx_tmp_cwd/dirname'.