Retain diff files as artefacts

Author: jamesrswift

.github/workflows/tests.yml

@@ -52,6 +52,16 @@ jobs:
       - name: Run test suite
         run: just test
 
+      - name: Archive diffs
+        uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: diffs
+          path: |
+            tests/**/diff/*.png
+            tests/**/out/*.png
+          retention-days: 5
+
       - name: Build docs
         if: ${{ matrix.doc }}
         run: just doc