ci: Run testing job whenever commits added to PR

[blukat29]

Make the testing job run when PR is opened, reopened, and commits added. (previously only when opened)

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows

By default, a workflow only runs when a pull_request event's activity type is opened, synchronize, or reopened.

https://github.com/orgs/community/discussions/24567

if you push a new commit to the HEAD ref of a pull request then this “synchronize” event will be triggered.