Use GitHub Action to upload diff images on test failure

Author: weiji14

.github/workflows/ci_tests.yaml

@@ -117,6 +117,12 @@ jobs:
         shell: bash -l {0}
         run: make test PYTEST_EXTRA="-r P"
 
+      # Upload diff images on test failure
+      - name: Upload diff images if any test fails
+        uses: actions/upload-artifact@v2
+        if: ${{ failure() }}
+          path: tmp-test-dir-with-unique-name
+
       # Build the documentation
       - name: Build the documentation
         shell: bash -l {0}