test harness: When the cardinality check fails, pretty-print \$expected_region_highlight and \$region_highlight.

tests/test-highlighting.zsh

@@ -212,6 +212,11 @@ run_test_internal() {
       details+="«$(typeset_p expected_region_highlight)» «$(typeset_p region_highlight)»"
       tap_escape $details; details=$REPLY
       print -r -- "not ok $i - cardinality check - $details${todo:+ - }$todo"
+      paste \
+        =(print -rC1 -- "expected_region_highlight" "${(qq)expected_region_highlight[@]}") \
+        =(print -rC1 -- "region_highlight" "${(qq)region_highlight[@]}") \
+        | if type column >/dev/null; then column -t -s $'\t'; else cat; fi \
+        | sed 's/^/# /'
     fi
   fi
 }