tests/divert: use PF_DIVERT

Now all Python ports has been patched to support PF_DIVERT, and
Python kinda promises to add support in 3.12 [1].

This reverts commit 322b5b7c16666c40d2763f18c1a51e6f0580d4e9.

[1] https://github.com/python/cpython/pull/96536#issuecomment-1303974686