bpo-35458: Fix test_shutil.test_disk_usage() (GH-11111)

The following test fails if a different process creates or removes
a file on the same disk partition between the two lines:

    usage = shutil.disk_usage(os.path.dirname(__file__))
    self.assertEqual(usage, shutil.disk_usage(__file__))

Only test that disk_usage() succeed on a filename, but don't check
the result. Add also tests on the fields type (must be int).