unittests: Fix mycrc32_combine unit test

Unit test for 'mycrc32_combine' did not checked 'MFSBLOCKSIZE'.
It was producing 'segmentation fault' for 'MFSBLOCKSIZE' lower than '65536'.
In particular using: 'cmake -DENABLE_LIGHTMFS:BOOL=YES' is using 'MFSBLOCKSIZE=4096'.