Commit 85635600 switched from
hardcoded use of strcontent to hardcoded use of fixedcontent
(fixedcontent is *sometimes* a copy of strcontent with a \n
appended). This was a problem because sometimes fixedcontent is *not*
a copy of strcontent, but is instead NULL, leading to the regenerated
test case output being a 0 length file.

This patch creates a new const char *cmpcontent initialized to
strcontent, but changed to fixedcontent if/when fixedcontent is
created, then always uses cmpcontent instead of (str|fixed)content.