Check that regenerating test data is idempotent (#85)

.github/workflows/generated_files.yml

@@ -0,0 +1,24 @@
+name: generatedfiles
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+permissions:
+  contents: read
+
+jobs:
+  generatedfiles_job:
+    runs-on: ubuntu-latest
+    name: Regenerate derived files
+    steps:
+      - name: Check out the repository to the runner
+        uses: actions/checkout@v4
+      - name: Regenerate the log test data
+        run: ./testdata/build_log.sh
+      - name: Confirm there are no diffs
+        run: git diff --exit-code