test: wait for native connection slot release

transport/native/server_test.go

@@ -862,6 +862,13 @@ func TestServerConnectionLimitReleasesAfterRequest(t *testing.T) {
 		t.Fatal(err)
 	}
 	firstResponse.Body.Close()
+	deadline := time.Now().Add(time.Second)
+	for server.Stats().ActiveConnections != 0 && time.Now().Before(deadline) {
+		time.Sleep(time.Millisecond)
+	}
+	if stats := server.Stats(); stats.ActiveConnections != 0 {
+		t.Fatalf("connection slot was not released after response: %+v", stats)
+	}
 
 	third, err := net.Dial("tcp", listener.Addr().String())
 	if err != nil {