Fix test build after d138598

Building the tests was giving some vague error message about a doubly-defined
symbol.

The solution is to define ShutdownRequested in test_bitcoin.cpp as well
so that init.cpp does not get pulled in.

src/test/test_bitcoin.cpp

@@ -84,3 +84,8 @@ void StartShutdown()
   exit(0);
 }
 
+bool ShutdownRequested()
+{
+  return false;
+}
+