Tests for cvSmooth -> tests for boxFilter

[savuor]

fixes #25448

Motivation

The obsolete function cvSmooth has two modes in which it calls cv::boxFilter() inside with and without normalization.
This function is covered by tests exactly for that modes.
This means that by replacing cvSmooth call by cv::boxFilter() we will leave the coverage untouched (but more obvious) and remove that obsolete function from tests.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• 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.
• The feature is well documented and sample code can be built with the project CMake

[asmorkalov]

👍