virt-aa-helper-test: only fails go to stdout by default

By Default (without -d) the tests will only print Failures.
So a log should follow general "no message is a good message" style.

But the testfw checks always emit the skip info to stdout. Instead
they should use the redirection that is controlled by -d.

This avoids mesages like the following to clutter the log:
  Skipping FW AAVMF32 test. Could not find /usr/share/AAVMF/AAVMF32_CODE.fd

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>

tests/virt-aa-helper-test

@@ -155,7 +155,7 @@ testfw() {
             -e "s,</os>,<loader readonly='yes' type='pflash'>$fwpath</loader></os>,g" "$template_xml" > "$test_xml"
         testme "0" "$title" "-r -u $valid_uuid" "$test_xml"
     else
-        echo "Skipping FW $title test. Could not find $fwpath"
+        echo "Skipping FW $title test. Could not find $fwpath" >$output
     fi
 }