test: Add unit test coverage for Init and Shutdown code

Currently this code is not called in unit tests. Calling should make it
possible to write tests for things like IPC exceptions being thrown during
shutdown.

src/common/args.cpp

@@ -589,6 +589,14 @@ void ArgsManager::AddHiddenArgs(const std::vector<std::string>& names)
     }
 }
 
+void ArgsManager::ClearArgs()
+{
+    LOCK(cs_args);
+    m_settings = {};
+    m_available_args.clear();
+    m_network_only_args.clear();
+}
+
 void ArgsManager::CheckMultipleCLIArgs() const
 {
     LOCK(cs_args);