tests: make fuzz tests (mostly) deterministic with fixed seed

src/test/fuzz/fuzz.cpp

@@ -102,6 +102,12 @@ void ResetCoverageCounters() {}
 
 void initialize()
 {
+    // By default, make the RNG deterministic with a fixed seed. This will affect all
+    // randomness during the fuzz test, except:
+    // - GetStrongRandBytes(), which is used for the creation of private key material.
+    // - Creating a BasicTestingSetup or derived class will switch to a random seed.
+    SeedRandomForTest(SeedRand::ZEROS);
+
     // Terminate immediately if a fuzzing harness ever tries to create a socket.
     // Individual tests can override this by pointing CreateSock to a mocked alternative.
     CreateSock = [](int, int, int) -> std::unique_ptr<Sock> { std::terminate(); };