test: support do-not-merge label

Support do-not-merge label for PR. PR can't be merged
if this label is set. The use cases of this label are
to prohibit PRs that are needs to be rebased or squashed.

Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>

Author: satoru-takeuchi

.github/workflows/check-do-not-merge.yaml

@@ -0,0 +1,17 @@
+name: Check Do Not Merge Label
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, labeled, unlabeled]
+
+jobs:
+  check-label:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check for do-not-merge label
+        run: |
+          if [[ "${{ contains(github.event.pull_request.labels.*.name, 'do-not-merge') }}" == "true" ]]; then
+            echo "::error::This PR has the 'do-not-merge' label and cannot be merged"
+            exit 1
+          fi
+          echo "No blocking labels found"