test/e2e: replace sleeps with event-driven waits in chaos/group/store tests (#5231)

* test/e2e: replace sleeps with event-driven waits in chaos/group/store tests

Replace 21 time.Sleep calls with deterministic waiting using
WaitForOutput, WaitForTCPReady, and a new WaitForTCPUnreachable helper.
Add CountOutput method for snapshot-based incremental log matching.

* test/e2e: validate interval and cap dial/sleep to remaining deadline in WaitForTCPUnreachable

test/e2e/framework/process.go

@@ -144,6 +144,30 @@ func waitForClientProxyReady(configPath string, p *process.Process, timeout time
 	return true
 }
 
+// WaitForTCPUnreachable polls a TCP address until a connection fails or timeout.
+func WaitForTCPUnreachable(addr string, interval, timeout time.Duration) error {
+	if interval <= 0 {
+		return fmt.Errorf("invalid interval for TCP unreachable on %s: interval must be positive", addr)
+	}
+	if timeout <= 0 {
+		return fmt.Errorf("invalid timeout for TCP unreachable on %s: timeout must be positive", addr)
+	}
+	deadline := time.Now().Add(timeout)
+	for {
+		remaining := time.Until(deadline)
+		if remaining <= 0 {
+			return fmt.Errorf("timeout waiting for TCP unreachable on %s", addr)
+		}
+		dialTimeout := min(interval, remaining)
+		conn, err := net.DialTimeout("tcp", addr, dialTimeout)
+		if err != nil {
+			return nil
+		}
+		conn.Close()
+		time.Sleep(min(interval, time.Until(deadline)))
+	}
+}
+
 // WaitForTCPReady polls a TCP address until a connection succeeds or timeout.
 func WaitForTCPReady(addr string, timeout time.Duration) error {
 	if timeout <= 0 {