workflows: add pr-auditor and test plans to PR templates

Created by: bobheadxi

This pull request adds test plans to all PR templates, and introduces a pr-auditor workflow to ensure test plans are provided. For more details see https://github.com/sourcegraph/sourcegraph/issues/30427 and https://docs.sourcegraph.com/dev/background-information/testing_principles

Test plan: created action should correctly set a status on this pull request.

Created by Sourcegraph batch change robert/pr-auditor-rollout.