Add unit tests for signals generated by ProcessNewBlock()

After a recent bug discovered in callback ordering in MainSignals,
this test checks invariants in ordering of
BlockConnected / BlockDisconnected / UpdatedChainTip signals

src/test/test_bitcoin.cpp

@@ -38,6 +38,12 @@ FastRandomContext insecure_rand_ctx(insecure_rand_seed);
 extern bool fPrintToConsole;
 extern void noui_connect();
 
+std::ostream& operator<<(std::ostream& os, const uint256& num)
+{
+    os << num.ToString();
+    return os;
+}
+
 BasicTestingSetup::BasicTestingSetup(const std::string& chainName)
 {
         SHA256AutoDetect();