tests: fix use of fixedcontent variable

tests: fix use of fixedcontent variable

Commit 8563560026d192c2cf047b550ffd468692245ed6 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.