test/system: fix a -Wsign-compare warning in compiling.

test/system: fix a -Wsign-compare warning in compiling.

test/system/st_rados_create_pool.cc: In function ‘std::__cxx11::string get_temp_pool_name(const char*)’:
test/system/st_rados_create_pool.cc:128:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   assert(ret < sizeof(poolname));
         ^

Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>