with a size larger than the underlying file worked on Windows.  It
does <wink>.  However, merely creating an mmap that way has the side
effect of growing the file on disk to match the specified size.  A
*later* test assumed that the file on disk was still exactly as it was
before the new "size too big" test was added, but that's no longer true.
So added a hack at the end of the "size too big" test to truncate the
disk file back to its original size on Windows.