ci(core): Retry sending data over TCP in case of error

The first attempt occasionally fails, probably due to a race with the
listener's startup.

Additionally, perform minor adjustments to the output written to stderr
to ensure it is properly flushed in the CI environment.

.github/workflows/scripts/run-tests-core.sh

@@ -37,15 +37,31 @@ log 'Searching index pattern via Kibana API'
 response="$(curl "http://${ip_kb}:5601/api/saved_objects/_find?type=index-pattern" -s -u elastic:testpasswd)"
 echo "$response"
 count="$(jq -rn --argjson data "${response}" '$data.total')"
-if [[ $count -ne 1 ]]; then
+if (( count != 1 )); then
 	echo "Expected 1 index pattern, got ${count}"
 	exit 1
 fi
 
 log 'Sending message to Logstash TCP input'
-echo 'dockerelk' | nc -q0 "$ip_ls" 5000
 
-sleep 1
+declare -i was_retried=0
+
+# retry for max 10s (5*2s)
+for _ in $(seq 1 5); do
+	if echo 'dockerelk' | nc -q0 "$ip_ls" 5000; then
+		break
+	fi
+
+	was_retried=1
+	echo -n 'x' >&2
+	sleep 2
+done
+if ((was_retried)); then
+	# flush stderr, important in non-interactive environments (CI)
+	echo >&2
+fi
+
+sleep 3
 curl -X POST "http://${ip_es}:9200/_refresh" -u elastic:testpasswd \
 	-s -w '\n'
 
@@ -53,7 +69,7 @@ log 'Searching message in Elasticsearch'
 response="$(curl "http://${ip_es}:9200/logstash-*/_count?q=message:dockerelk&pretty" -s -u elastic:testpasswd)"
 echo "$response"
 count="$(jq -rn --argjson data "${response}" '$data.count')"
-if [[ $count -ne 1 ]]; then
+if (( count != 1 )); then
 	echo "Expected 1 document, got ${count}"
 	exit 1
 fi