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).

Refactor test runner.py with a saner unittest-based class hierarchy so
that --parallel args affects all test suites (all, by-name, failed).
Also change Makefile which now can be used like this:

make test-process ARGS=--parallel