✅ Improve test robustness

Optional file uploads with validation errors remain blank and don't
block form validation/submission, so while there were issues with the
upload, the test didn't fail.

Now the test fails if the upload didn't succeed.

src/openforms/tests/e2e/test_file_upload.py

@@ -37,6 +37,9 @@ def setUpTestData():
                             "key": "fileUpload",
                             "label": "File Upload",
                             "storage": "url",
+                            "validate": {
+                                "required": True,
+                            },
                         }
                     ]
                 },