python: how to unittest a method that calculates the size of a dir?

The following function calculates the size (bytes) of a given directory and its sub-directories

def get_dir_size(dir_path):
    """Determine the size of a dir.

    This function also takes into account the allocated size of
    directories (4096 bytes).

    Note: The function may crash on symlinks with something like:
    OSError: [Errno 40] Too many levels of symbolic links

    :param dir_path (str): path to the directory
    :return: size in bytes.
    """
    tot_size = 0
    for (root_path, dirnames, filenames) in os.walk(dir_path):
        for f in filenames:
            fpath = os.path.join(root_path, f)
            tot_size += os.path.getsize(fpath)
        tot_size += os.path.getsize(root_path)
    return tot_size

I want to write a unittest that ensures the function is behaving as expected. To ensure the correct outcome I assume a have to mock the directory I'll pass to the function. testfixtures looked promising bur turned out to create dirs and files in the /tmp directory.

In [5]: from testfixtures import TempDirectory
In [8]: d = TempDirectory()
In [14]: d.makedir('output')
Out[14]: '/tmp/tmpLWnsCr/output' 

What would be the right approach here?