`site.{usercustomize,sitecustomize}` hooks are never tested

[sobolevn]

Bug report

They are documented:

The Customization Modules
-------------------------

Python provides two hooks to let you customize it: :mod:`sitecustomize` and
:mod:`usercustomize`.  To see how it works, you need first to find the location
of your user site-packages directory.  Start Python and run this code::

   >>> import site
   >>> site.getusersitepackages()
   '/home/user/.local/lib/python3.5/site-packages'

Now you can create a file named :file:`usercustomize.py` in that directory and
put anything you want in it.  It will affect every invocation of Python, unless
it is started with the :option:`-s` option to disable the automatic import.

:mod:`sitecustomize` works in the same way, but is typically created by an
administrator of the computer in the global site-packages directory, and is
imported before :mod:`usercustomize`.  See the documentation of the :mod:`site`
module for more details.

But, they are not tested anywhere in the CPython.
I think that adding these test might be worth it, in case the setup is reliable.

Linked PRs

• gh-108747: Add unit tests for site.{usercustomize,sitecustomize} hooks #109470