Random Windows test failures on AppVeyor

[embray]

Several of the AppVeyor test runs are randomly erroring out in get_test_subprocess with errors like:

======================================================================
ERROR: setUpClass (test_windows.WindowsSpecificTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\projects\psutil\psutil\tests\test_windows.py", line 68, in setUpClass
    cls.pid = get_test_subprocess().pid
  File "c:\projects\psutil\psutil\tests\__init__.py", line 209, in get_test_subprocess
    os.remove(TESTFN)
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\projects\\psutil\\$testfile'

I'm just making this placeholder issues for it to say that I'm working on fixing this; will add a PR when I'm fairly confident it's fixed.

[giampaolo]

Oh yeah. That's a race condition / timing issue.
We need to first safe_remove() the file before running the subprocess, then retry a bunch of times in case of error == 32.
A better alternative would be to use some other synchronization mechanism other than a file, like a signal, but I'm not sure whether that can be done on Windows as historically it implements signals very poorly.

[embray]

Instead of safe_remove I'm using wait_for_file which I modified slightly to allow for this case as well.

[giampaolo]

I suppose this is fixed by now. Closing.