txgraph: Add simulation fuzz test (tests)

This adds a simulation fuzz test for txgraph, by comparing with a naive
reimplementation that models the entire graph as a single DepGraph, and
clusters in TxGraph as connected components within that DepGraph.

Author: sipa

src/test/fuzz/CMakeLists.txt

@@ -124,6 +124,7 @@ add_executable(fuzz
   tx_in.cpp
   tx_out.cpp
   tx_pool.cpp
+  txgraph.cpp
   txorphan.cpp
   txrequest.cpp
   # Visual Studio 2022 version 17.12 introduced a bug