[Ada][testsuite] use sync to avoid missing executable

[Florian Weimer]

* Laurent GUERBY:

> +      if [ ! -x $dir/tests/$chapter/$i/$binmain ]; then
> +         sync
> +      fi

Are you sure you need the sync, and not just the delay introduced by
it?  I agree with Arno that it's a kernel bug if you really need it.
sync shouldn't have any observable effect (except for the delay, of
course).