size_t is not always an unsigned long on all platforms, but for diagnostic
message in a test, it should be fine to cast it this way
addresses these warnings:
unittest/libmariadb/connection.c:635:3: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'time_t' [-Wformat=]
diag("elapsed: %u", elapsed);
unittest/libmariadb/dyncol.c:149:5: warning: format '%d' expects argument of type 'int', but argument 3 has type 'size_t' [-Wformat=]
diag("%s %d", my_keys[i].str, my_keys[i].length);
unittest/libmariadb/dyncol.c:227:5: warning: format '%d' expects argument of type 'int', but argument 3 has type 'size_t' [-Wformat=]
diag("Key: %s Len: %d", unpack_keys[i].str, unpack_keys[i].length);
