Having fixed several bugs relating to flushing of FILE* streams (with
fflush and other operations) and their offsets (both the file position
indicator in the FILE*, and the offset in the underlying open file
description), especially after ungetc but not limited to that case,
add a test that more systematically covers different combinations of
cases for such issues, with 57220 separate scenarios tested (which
include examples of all the five separate fixed bugs), all of which
pass given the five previous bug fixes.
Tested for x86_64.
