Add an expected-file to match behavior of latest libxml2.

Recent releases of libxml2 do not provide error context reports for errors
detected at the very end of the input string.  This appears to be a bug, or
at least an infelicity, introduced by the fix for libxml2's CVE-2015-7499.
We can hope that this behavioral change will get undone before too long;
but the security patch is likely to spread a lot faster/further than any
follow-on cleanup, which means this behavior is likely to be present in the
wild for some time to come.  As a stopgap, add a variant regression test
expected-file that matches what you get with a libxml2 that acts this way.