test harness: Rewrite the columnar pretty-printer without external tools.

tests/test-highlighting.zsh

@@ -230,10 +230,7 @@ run_test_internal() {
         if (( difference > 0 )); then
           left_column+=( ${(r:2*difference::. :):-} )
         fi
-        paste \
-          =(print -rC1 -- $left_column) \
-          =(print -rC1 -- $right_column) \
-          | if type column >/dev/null; then column -t -s $'\t'; else cat; fi \
+        print -rC2 -- "${left_column[@]}" "${right_column[@]}" \
           | sed 's/^/# /'
       }
     fi