Fix monitoring page registration races in tests (#8770)

Author: snaury

ydb/core/mon/sync_http_mon.cpp

@@ -54,11 +54,8 @@ namespace NActors {
     }
 
     void TSyncHttpMon::Stop() {
-        if (IndexMonPage) {
-            IndexMonPage->ClearPages(); // it's required to avoid loop-reference
-            TBase::Stop();
-            IndexMonPage.Drop();
-        }
+        IndexMonPage->ClearPages(); // it's required to avoid loop-reference
+        TBase::Stop();
     }
 
     void TSyncHttpMon::Register(NMonitoring::IMonPage* page) {

ydb/core/testlib/actors/test_runtime.cpp

@@ -27,7 +27,6 @@ namespace NActors {
     void TTestActorRuntime::TNodeData::Stop() {
         if (Mon) {
             Mon->Stop();
-            GetAppData<NKikimr::TAppData>()->Mon = nullptr;
         }
         TNodeDataBase::Stop();
     }