Revisions and improvements to PR workflows

1. Update workflows to reduce the amount of tests that are triggered (should only be a sniff test, not hours of test execution).
2. Check that the Github runners are being set up properly (with up-to-date prereqs, jtreg version, etc).
3. Trigger workflow after pushing changes to a PR
- We limited automatically triggered PR testing to `ready_for_review` (see: https://github.com/AdoptOpenJDK/openjdk-tests/blob/master/.github/workflows/directoriesFilesChangePR.yml#L4).
- If people do not create a draft, the workflow does not get triggered. The workflow should be triggered after pushing changes to a PR. Maybe we should use `synchronize `. We should check the gitbub docs.

Start with .github/workflows/directoriesFilesChangePR.yml and the GitHub documentation for pull_request events, especially synchronize and ready_for_review. Review the runner prerequisites and jtreg version, then verify that PR changes trigger only the intended sniff test and that runner setup uses current prerequisites.

Written by the indexing model from the issue text.