ci: respdiff -display stderr log on failure

(cherry picked from commit 3a1e5626)

.gitlab-ci.yml

@@ -257,6 +257,7 @@ test:linux:amd64:valgrind:
       $(sudo -u respdiff /var/opt/respdiff/contrib/job_manager/create.py
       "$(git rev-parse --short HEAD)" -l $LABEL -t $RESPDIFF_TEST
       --knot-branch 2.7)
+    - for f in $TESTDIR/*.json; do test -s "$f" || (cat $TESTDIR/*stderr*; exit 1); done
     - sudo -u respdiff /var/opt/respdiff/contrib/job_manager/plot_ref.sh $TESTDIR/.. /var/tmp/respdiff-jobs/ref_current $RESPDIFF_TEST
   after_script:
     - 'cp -t . respdiff_commitdir/$RESPDIFF_TEST/j* ||:'