txgraph: Add DoWork function (feature)

This can be called when the caller has time to spend now, and wants future operations
to be fast.

src/test/fuzz/txgraph.cpp

@@ -526,6 +526,10 @@ FUZZ_TARGET(txgraph)
                 // these here without making more calls to real, which could affect its internal
                 // state. A full comparison is done at the end.
                 break;
+            } else if (command-- == 0) {
+                // DoWork.
+                real->DoWork();
+                break;
             }
         }
     }