tests: Use fixedcontent variable

Commit ca32929908bbc94116493ad8915e7cd7ae0f57d5 added function
virTestCompareToFile(), but forgot to use a fixedcontent value for the
actual comparison.  That lead to VIR_TEST_DEBUG=1 showing (for some
tests) all the actual output from the first error to the end of the
string due to the difference being an endline in the end.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

tests/testutils.c

@@ -709,7 +709,7 @@ virTestCompareToFile(const char *strcontent,
                         filecontent)) {
         virTestDifferenceFull(stderr,
                               filecontent, filename,
-                              strcontent, NULL);
+                              fixedcontent, NULL);
         goto failure;
     }