Commit ca329299 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: NMartin Kletzander <mkletzan@redhat.com>