GH Action: try testing the action from a GH Action

Based on notes from https://dev.to/cicirello/how-to-test-a-github-action-with-github-actions-2hag

.github/workflows/branch-builder.yml

@@ -0,0 +1,23 @@
+name: Branch Builder
+
+on:
+    pull_request:
+      types: [opened, reopened, synchronize]
+    push:
+      branches: [ main ]
+      tags-ignore: "**"
+    workflow_dispatch:
+env:
+  BRANCH_NAME: ${{ github.head_ref || github.ref_name }} # github.head_ref for pull_request event and github.ref_name for push event
+  SLACK_BOT_TOKEN: ${{ secrets.SLACK_TEST_TOKEN }}
+  SLACK_CHANNEL_ID: ${{ secrets.SLACK_TEST_CHANNEL_ID }}
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Test Slack message with default settings
+        uses: ./
+        with:
+          channel: ${{ env.SLACK_CHANNEL_ID }}
+          text: "branch-builder message from `ynab/slack-post-message-action/${{ env.BRANCH_NAME }}`:\n\n*Testing!*\n\nhttps://www.ynab.com"