Add layer model blend test pictures

[jsxyhelu]

No description provided.

[jsxyhelu]

from opencv/opencv#17520
Because my test here needs to call related image resources, according to the current practice, they are placed in opencv_extra (https://github.com/opencv/opencv_extra/pull/766）. In this case, you must first pass in extra, and the build-bot here can succeed. Please help me to promote This question.
@mshabunin

[jsxyhelu]

What should I do? First of all, upload the image to extra, and then opencv test can pass, but the extra here can't pass.

[alalek]

For related patches you should create branches with the same name:

add_layer_model_blend
add_layer_model_blending

Otherwise there is no reliable logic how to find that.

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(currently it is fixed manually, added opencv_extra= instruction to the main PR)

P.S. In case of updating extra's PR, you should re-trigger builds of the main PR. So you need to push updated/amended commits to the main PR after the pushed update to opencv_extra.

[jsxyhelu]

@alalek thanks,I will pay attention to keeping the names of both sides consistent next time