I looked for any existing tests (by grepping for 'signals' and 'import' on the same line), didn't find any (because my assumption that 'signals' would have to be explicitly imported was wrong).

Wrote some new tests, which made me also make some further changes to unittest/signals.py: https://gist.github.com/mgedmin/a91872054884dbaaa344

And that's how I discovered that there's an existing test suite for SIGINT handling, in test_break.py, and that it fails after my changes.

Stay tuned.