test: fix Windows compiler warnings in overlapped-checker
Fixes two warnings:
test\overlapped-checker\main_win.c(37,25): warning : format specifies type 'int' but the argument has type 'DWORD' (aka 'unsigned long') [-Wformat]
test\overlapped-checker\main_win.c(9,14): warning : unused variable 'write_count' [-Wunused-variable]