Synchronize SIGINT delivery in test_general_signal

[ryanchou1994]

test_general_signal currently relies on two-second sleeps in the child. If the output callback is delayed after reading 3, the child can print 4 before SIGINT arrives, breaking the expected 0, 1, 2, 3, 42, 43 output.

Wait for the handler's existing i = 42 update before printing the final two values. The wait uses a monotonic deadline and a short sleep; the handler only assigns the variable. A parent timeout also bounds the test if the child stops making progress. The change is confined to this test.

Validation:

• A controlled three-second callback delay fails on the upstream version and passes with this change on macOS and Linux. Delays of zero and eight seconds also pass.
• A signal arriving before the wait succeeds; no signal produces a timeout error after ten seconds.
• macOS, Python 3.14.6: 188 passed, 2 skipped.
• Linux, Python 3.14.7: poll 188 passed, 2 skipped; select 187 passed, 3 skipped; the root test passes with both backends. The changed test also passes under the C locale with both backends.
• Ruff check and format pass using the version in uv.lock.

Refs #784.

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