fuzz: DisableNextWrite

This is required in the process_message(s) fuzz targets to avoid leaking
the next write time from one run to the next. Also, disable it
completely because it is not needed and due to leveldb-internal
non-determinism.

src/test/fuzz/process_message.cpp

@@ -65,6 +65,7 @@ FUZZ_TARGET(process_message, .init = initialize_process_message)
     auto& chainman = static_cast<TestChainstateManager&>(*g_setup->m_node.chainman);
     SetMockTime(1610000000); // any time to successfully reset ibd
     chainman.ResetIbd();
+    chainman.DisableNextWrite();
 
     node::Warnings warnings{};
     NetGroupManager netgroupman{{}};