change fcn8s-heavy-pascal tests from caffe to onnx

[WanliZhong]

Merged with: https://github.com/opencv/opencv_extra/pull/1171 Part of https://github.com/opencv/opencv/issues/25314

This PR aims to change the tests related to fcn8s-heavy-pascal from caffe framework to onnx. The new fcn models have different backbone and output.

new model link: https://github.com/onnx/models/blob/main/validated/vision/object_detection_segmentation/fcn/model/fcn-resnet50-12.onnx

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• [x] I agree to contribute to the project under Apache 2 License.
• [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
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• [ ] There is a reference to the original bug report and related work
• [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Patch to opencv_extra has the same branch name.
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[WanliZhong]

CI failed because of the lack of new model.

[WanliZhong]

PR should target 4.x branch

Done.