Automate merge Dependabot PRs for tests (#416)

* Enable automerge dependabot
* Move checking condition in auto merge for auto approve.

.github/dependabot-auto-merge.yml

@@ -0,0 +1,33 @@
+# References:
+# * https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions
+# * https://github.com/dependabot/fetch-metadata
+
+name: Dependabot auto-merge
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v1
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+      - name: Approve a PR
+        if: ${{contains(steps.dependabot-metadata.outputs.directory, '/test')}}
+        run: gh pr review --approve "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
+      - name: Enable auto-merge for Dependabot PRs
+        if: success()
+        run: gh pr merge --auto --merge "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}