adds observer_pattern tests

tests/observer_pattern/observer/test_observer.py

@@ -0,0 +1,25 @@
+from typing import Any
+
+import pytest
+from pydase.observer_pattern.observable import Observable
+from pydase.observer_pattern.observer import Observer
+
+
+def test_abstract_method_error() -> None:
+    class MyObserver(Observer):
+        pass
+
+    class MyObservable(Observable):
+        pass
+
+    with pytest.raises(TypeError):
+        MyObserver(MyObservable())
+
+
+def test_constructor_error() -> None:
+    class MyObserver(Observer):
+        def on_change(self, full_access_path: str, value: Any) -> None:
+            pass
+
+    with pytest.raises(TypeError):
+        MyObserver()