Speed up test suite by avoiding fsync

Speed up test suite by avoiding fsync

Add grub_util_disable_fd_syncs call to turn grub_util_fd_sync calls into
no-ops, and use it in programs that copy files but do not need to take
special care to sync writes (grub-mknetdir, grub-rescue,
grub-mkstandalone).

On my laptop, this reduces partmap_test's runtime from 1236 seconds to
204 seconds.