Eliminate the need of ::testsing::ValuesIn() for CV_ENUM

Also cv::, cv::gpu:: and cv::ocl:: namespace prefixes can be safely omitted
inside CV_ENUM and CV_FLAGS
Let wave_size=1 on CPU.
Add clamping for y dimension.
Merge pull request #822 from taka-no-me:better_cv_enum
Merge pull request #825 from pengx17:2.4_query_cpu_wavesize
Merge pull request #826 from pengx17:2.4_canny_clampfix