ci: Upload result images if tests fail.

Author: QuLogic

.github/workflows/tests.yml

@@ -222,3 +222,9 @@ jobs:
         if: ${{ runner.os != 'macOS' }}
       - name: Upload code coverage
         uses: codecov/codecov-action@v1
+
+      - uses: actions/upload-artifact@v2
+        if: failure()
+        with:
+          name: "${{ matrix.python-version }} ${{ matrix.os }} ${{ matrix.name-suffix }} result images"
+          path: ./result_images