Add a reusable `psutil_getpagesize()` utility common to all UNIXes.

Related to #1885 (`getpagesize()` is deprecated on recent macOS, POSIX.1-2001 and possibly other UNIXes).

The problem emerged on macOS but `getpagesize()` is also used in FreeBSD, NetBSD, OpenBSD and AIX, so it makes sense to do this in one place only, similarly to Windows which also provide a `psutil_getpagesize()` utility.

Follow cPython's `mmapmodule.c` and `resourcemodule.c` lead and rely on `sysconf(_SC_PAGESIZE)` instead, but leave `getpagesize()` in place as last resort/attempt for systems where it's not deprecated and/or they still legitimately rely on it.

Also provide a python wrapper so we can test the return value of this C function against Python's stdlib modules.

Signed-off-by: Giampaolo Rodola <g.rodola@gmail.com>

...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).