I don't think you should expect every bug to be caught by unit testing. But where it helps with a problem like that is eliminating a lot of other possible causes of bugs. Debugging something like this is often a needle-in-a-haystack problem, but it's nice if you can rule out most of the hay from the beginning.

In this case, once I discovered the cause of the bug I would have written a unit test that exposed it, probably a very focused one. Then I would have gone hunting for other missed opportunities to test for this, and I imagine my team would have come up with some sort of general rule for testing signal handlers.