Further improve `test_future_stmt`

[sobolevn]

Bug report

Based on the review of @vstinner in #109368 (review)

I propose to:

1. Replace badsyntax_future files with regular code samples, so we can more easily manage them
2. Keep at lest one file to be sure that it also works as intended for modules
3. Apply better names

I have a PR ready :)

Linked PRs

• gh-109485: Further improve test_future_stmt tests #109486