Stop concurrent CI test workflow runs

Author: tadayosi

.github/workflows/test.yml

@@ -12,6 +12,10 @@ on:
     paths-ignore:
       - "**.md"
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: ubuntu-latest