Both are called at different points when running './python.exe -m test -j3':
1. In __main__.py, this is called once at the start of the test run. By putting TESTCWD as the first/name arg, the directory is created.
2. In regrtest.py, the main file is called per test.

The support.temp_cwd command was raising a warning if the directory already existed. I've updated this to no longer raise a warning, although it will still try to create it. Passing it in explicitly as the path= argument would have it not try to create the directory.