minor: tests: Add comment for NONE in expected_region_highlight

tests/test-highlighting.zsh

@@ -110,6 +110,7 @@ run_test_internal() {
     integer start=$highlight_zone[1] end=$highlight_zone[2]
     # Escape # as ♯ since the former is illegal in the 'description' part of TAP output
     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 $highlight_zone[4]"
     for j in {$start..$end}; do