Ensure tests have default-initialized settings (#4498)

Also rework HighlightController test directory creation/saving to ensure the test directory is written to & cleaned up appropriately

src/singletons/Settings.cpp

@@ -140,6 +140,8 @@ Settings::Settings(const QString &settingsDirectory)
 
 Settings &Settings::instance()
 {
+    assert(instance_ != nullptr);
+
     return *instance_;
 }