Add some tests for NotebookTab (#5070)

* EmptyApplication: Add asserts to rest of getters (except for getSeventvAPI)

* Theme: make getTheme call getIApp()->getThemes() instead

this allows it to be used in tests
realistically this should be deprecated & users of it should just call
getIApp()->getThemes() directly

* Use getIApp() instead of getApp() in a few places

src/singletons/Theme.cpp

@@ -476,7 +476,7 @@ void Theme::normalizeColor(QColor &color) const
 
 Theme *getTheme()
 {
-    return getApp()->themes;
+    return getIApp()->getThemes();
 }
 
 }  // namespace chatterino