Too easy for 6 kyu.

There are warnings in sample test from C compiler:

fixture.c:104:14: warning: passing 'char []' to parameter of type 'unsigned char *' converts between pointers to integer types with different sign [-Wpointer-sign]
  imageTest (codewars_rgb,
             ^~~~~~~~~~~~
fixture.c:18:32: note: passing argument to parameter 'imageData' here
void imageTest (unsigned char *imageData, int h, int w, void* weights, int n, const unsigned char *expected) {
                               ^
fixture.c:107:14: warning: passing 'char []' to parameter of type 'const unsigned char *' converts between pointers to integer types with different sign [-Wpointer-sign]
             codewars_laplacianFiltered);
             ^~~~~~~~~~~~~~~~~~~~~~~~~~
fixture.c:18:100: note: passing argument to parameter 'expected' here
void imageTest (unsigned char *imageData, int h, int w, void* weights, int n, const unsigned char *expected) {
                                                                                                   ^
2 warnings generated.

It may be fixed by explicit conversion to (unsigned char *):

  imageTest ((unsigned char *)codewars_rgb,
             codewars_height, codewars_width,
             kernel_laplacianFilter, 9,
             (unsigned char *)codewars_laplacianFiltered);

Seems too easy for 5 kyu.