test: wait for native request slot release

transport/native/server_test.go

@@ -785,6 +785,13 @@ func TestServerConcurrentRequestLimitRecovers(t *testing.T) {
 	if response.StatusCode != http.StatusOK {
 		t.Fatalf("expected held request to complete, got %d", response.StatusCode)
 	}
+	deadline := time.Now().Add(time.Second)
+	for server.Stats().ActiveRequests != 0 && time.Now().Before(deadline) {
+		time.Sleep(time.Millisecond)
+	}
+	if stats := server.Stats(); stats.ActiveRequests != 0 {
+		t.Fatalf("request slot was not released after response: %+v", stats)
+	}
 
 	thirdConn, err := net.Dial("tcp", listener.Addr().String())
 	if err != nil {