txgraph: Add internal sanity check function (tests)

To make testing more powerful, expose a function to perform an internal sanity
check on the state of a TxGraph. This is especially important as TxGraphImpl
contains many redundantly represented pieces of information:

* graph contains clusters, which refer to entries, but the entries refer back
* graph maintains pointers to Ref objects, which point back to the graph.

This lets us make sure they are always in sync.

src/test/fuzz/txgraph.cpp

@@ -363,6 +363,11 @@ FUZZ_TARGET(txgraph)
             }
         }
     }
+
+    // After running all modifications, perform an internal sanity check (before invoking
+    // inspectors that may modify the internal state).
+    real->SanityCheck();
+
     // Compare simple properties of the graph with the simulation.
     assert(real->GetTransactionCount() == sim.GetTransactionCount());
 
@@ -411,6 +416,9 @@ FUZZ_TARGET(txgraph)
         }
     }
 
+    // Sanity check again (because invoking inspectors may modify internal unobservable state).
+    real->SanityCheck();
+
     // Remove all remaining transactions, because Refs cannot be destroyed otherwise (this will be
     // addressed in a follow-up commit).
     for (auto i : sim.graph.Positions()) {