test: Add SyncWithValidationInterfaceQueue to mockscheduler RPC

This makes existing tests less brittle, see
https://github.com/bitcoin/bitcoin/pull/28108/files#r1268966663

Author: MarcoFalke

src/rpc/node.cpp

@@ -93,6 +93,7 @@ static RPCHelpMan mockscheduler()
     // protect against null pointer dereference
     CHECK_NONFATAL(node_context->scheduler);
     node_context->scheduler->MockForward(std::chrono::seconds(delta_seconds));
+    SyncWithValidationInterfaceQueue();
 
     return UniValue::VNULL;
 },