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

.github/PULL_REQUEST_TEMPLATE.md

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+### Test plan
+
+<!--
+  As part of SOC2/GN-104 and SOC2/GN-105 requirements, all pull requests are REQUIRED to
+  provide a "test plan". A test plan is a loose explanation of what you have done or
+  implemented to test this, as outlined in our Testing principles and guidelines:
+  https://docs.sourcegraph.com/dev/background-information/testing_principles
+  Write your test plan here after the "Test plan" header.
+-->

.github/workflows/pr-auditor.yml

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+name: pr-auditor
+on:
+  pull_request:
+    types: [ closed, edited, opened ]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with: { repository: 'sourcegraph/sourcegraph' }
+      - uses: actions/setup-go@v2
+        with: { go-version: '1.17' }
+
+      - run: ./dev/pr-auditor/check-pr.sh
+        env:
+          GITHUB_EVENT_PATH: ${{ env.GITHUB_EVENT_PATH }}
+          GITHUB_TOKEN: ${{ secrets.CODENOTIFY_GITHUB_TOKEN }}
+          GITHUB_RUN_URL: https://github.com/sourcegraph/infrastructure/actions/runs/${{ github.run_id }}