bpo-47168: List feature macros in the stable ABI manifest, improve tests

[encukou]

This depends on #32414. The refactoring+tests should go in a separate commit.

I plan some more deduplication, and use the name “feature macro” rather than ifdef everywhere, after converting stable-abi.txt to TOML so it's easier to work with.

https://bugs.python.org/issue47168

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 896f4d0 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.