tests: Use idiomatic set check

tests/test-highlighting.zsh

@@ -137,7 +137,7 @@ run_test_internal() {
     local desc="[$start,$end] «${BUFFER[$start,$end]//'#'/♯}»"
     # Match the emptiness of observed_result if no highlighting is expected
     [[ $highlight_zone[3] == NONE ]] && highlight_zone[3]=
-    [[ -n "$highlight_zone[4]" ]] && todo="# TODO $highlight_zone[4]"
+    (( $+highlight_zone[4] )) && todo="# TODO $highlight_zone[4]"
     for j in {$start..$end}; do
       if [[ "$observed_result[$j]" != "$highlight_zone[3]" ]]; then
         print -r -- "not ok $i - $desc - expected ${(qqq)highlight_zone[3]}, observed ${(qqq)observed_result[$j]}. $todo"