refactored filters accuracy tests in the ocl module;

- refactored accuracy tests in ocl/test/test_filters.cpp
- added an accuracy test for ocl::medianFilter
- fixed Filter2D in case of CV_32FC3
- removed needless piece of code from ocl::erode
- marked bottleneck in tests as // TODO ..
- fixed ocl::Laplacian (when used no default scale=1)