Pull Request Readiness Checklist

In test TEST(Core_SolvePoly, regression_5599) function checkRoot is used for checking that obtained roots contain all expected roots. To properly check multiple roots (roots with multiplicity greater than 1) it should check that some root is presented exactly several times. For this in function there is logic to assign found roots to NaN but it didn't work because it used root copy instead of reference for it.

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• 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