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
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