test: add trailing commas in event tests

As much as I would like to do this everywhere and then modify the lint rule to enforce it, the churn would be too big. However if we're going to have relatively frequent nits for this sort of thing (as we do), I'd prefer we migrate a few files at a time to never actually getting around to doing it.

Ref: https://github.com/nodejs/node/pull/45448#pullrequestreview-1179370442