lightning: fix test lightning_reload_cert (#42805)

close #42807

Author: sleepymole

br/tests/lightning_disable_scheduler_by_key_range/run.sh

@@ -29,7 +29,7 @@ shpid="$!"
 sleep 15
 ok=0
 for _ in {0..60}; do
-  if grep -Fq "connection error" "$TEST_DIR"/lightning.log; then
+  if grep -Fq "connection closed before server preface received" "$TEST_DIR"/lightning.log; then
     ok=1
     break
   fi