Custom test runner to avoid linking libpython

[gatesn]

Currently we link libpython in order to run Zig tests that spawn python.
We then launch these Zig tests from Pytest in order to run all tests together.

This proposal is to invert the control flow by building a custom Zig test runner that gets built as a shared library. This library is then loaded into Pytest as a native extension and executed that way.

This means we can use Pythons that have been statically linked to libpython, and our startup time is probably faster. We should probably still fork/exec in order to isolate segfaults though.