Use compatible sprintf format for size_t

[srubin-nottinghamspirk]

In test_common.c, sprintf uses the "%i" formatter for a size_t. On certain compilers (i.e. gcc 13.2.0 on my build machine), this causes a warning. In CMakeLists.txt, warnings are set to be errrors (-Werror), so the code will not build. The fix is to use "%zu" for ISO C99 compatibility.