visual tests: skip visual tests when test data are not present

test/run

@@ -24,11 +24,15 @@ if [ -n "$(find test/standalone/ -maxdepth 1 -name '*-bin' -print -quit)" ]; the
     done
 fi
 
+if [ -d "test/data-visual/styles" ]; then
     run_substep "Running visual tests..."
     if [ -z "$JOBS" ]; then
         JOBS=1
     fi
     ./test/visual/run -j $JOBS
     failures=$((failures+$?))
+else
+    echo "Notice: Skipping visual tests, the visual tests data are not present under the standard directory \"test/data-visual\"."
+fi
 
 exit $failures