Fix testHAProxyModeConnectionWorks is flaky (opensearch-project#16087)

* Fix testHAProxyModeConnectionWorks is flaky

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

* Optimize health check method

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

---------

Signed-off-by: Gao Binlong <gbinlong@amazon.com>

Author: gaobinlong

qa/remote-clusters/docker-compose.yml

@@ -72,3 +72,11 @@ services:
       - "9600"
     volumes:
       - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro
+    depends_on:
+      - opensearch-2
+    healthcheck:
+      test: ["CMD", "timeout", "1", "bash", "-c", "cat < /dev/null > /dev/tcp/localhost/9600"]
+      interval: 2s
+      timeout: 1s
+      retries: 5
+      start_period: 15s