CI: add coverage reporting, bump deprecated action versions

Increasing Access

First - there's no coverage tracking at all. No --coverage flag, no collectCoverage in the jest config, nothing. We've got ~55 test files and zero idea what they actually cover. Would be good to at least get text-summary output in CI so we have a number to look at. Doesn't need to be fancy, just run jest with --coverage and maybe set a low global threshold so it doesn't silently tank when someone guts a test file

Second - test.yml is still on actions/checkout@v2 and actions/setup-node@v1. Both have been deprecated for ages, they run on Node 12 internally, and GitHub's been spamming warnings in the action logs about it. Should be @v4 for both. Two-line change

Feature enhancement details

Neither of these is blocking anything, but it's low-hanging fruit and the coverage thing especially would be nice to have before more test work gets piled on.,.