test harness: Fix the pretty-printer's padding implementation.

The new implementation is less efficient but definitely correct.

tests/test-highlighting.zsh

@@ -227,9 +227,7 @@ run_test_internal() {
         left_column=( "expected_region_highlight" "${(qq)expected_region_highlight[@]}" )
         right_column=( "region_highlight" "${(qq)region_highlight[@]}" )
         integer difference=$(( $#right_column - $#left_column ))
-        if (( difference > 0 )); then
-          left_column+=( ${(r:2*difference::. :):-} )
-        fi
+        repeat $difference do left_column+=(.); done
         paste \
           =(print -rC1 -- $left_column) \
           =(print -rC1 -- $right_column) \