...in order to accomodate Cygwin implementation.

Test files/dirs are now removed after each test. when invoked via self.get_testfn().
Until now test files were stored in a global variable and were removed at process exit, via atexit.register(), but this didn't work with parallel tests because the fork()ed workers use os._exit(0), preventing cleanup functions to run.
All test classes now inherit from PsutilTestCase class, which provides the most important methods requiring an automatic cleanup (get_test_subprocess() and others).

On a second thought I realized that process_iter() uses a global
variable, so it's not thread safe.
That means that if the are 2 threads using it, the first thread one calling the function (+ consume the iterator), will "steal" the processes of the second thread.

psutil.cpu_percent() has the same problem. That means we have a problem
can't solve with the current API and requires a lot of thinking on how
to solve it as it's not obvious.