Unit test for 64-bit, countnonZero, Mat.dot, split, merge

[lionkunonly]

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 OpenCV (BSD) License.
• [ ] To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• [ ] The PR is proposed to proper branch
• [ ] There is reference to 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

[lionkunonly]

340

[lionkunonly]

Thanks for your suggestions. I will complete the suggestion 2 firstly.

[huningxin]

Thanks for your suggestions. I will complete the suggestion 2 firstly.

Sounds good. Once you change this PR, feel free to ping me for review.