Merge pull request #27331 from MaximSmolskiy:add-test-for-solveCubic

Add tests for solveCubic #27331

### Pull Request Readiness Checklist

Related to #27323 

I found only randomized tests with number of roots always equal to `1` or `3`, `x^3 = 0` and some simple test for Java and Swift.
Obviously, they don't cover all cases (implementation has strong branching and number of roots can be equal to `-1`, `0` and `2` additionally).
So, I think it will be useful to try explicitly cover more cases (and implementation branches correspondingly)

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

- [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
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/core/src/mathfuncs.cpp

@@ -1590,7 +1590,7 @@ int cv::solveCubic( InputArray _coeffs, OutputArray _roots )
     {
         if( a1 == 0 )
         {
-            if( a2 == 0 )
+            if( a2 == 0 ) // constant
                 n = a3 == 0 ? -1 : 0;
             else
             {
@@ -1624,6 +1624,7 @@ int cv::solveCubic( InputArray _coeffs, OutputArray _roots )
     }
     else
     {
+        // cubic equation
         a0 = 1./a0;
         a1 *= a0;
         a2 *= a0;