Add test for singleton pattern

tests/test_chapter_05/test_singleton_pattern.py

@@ -0,0 +1,15 @@
+"""Module for testing pattern "Singleton"."""
+
+
+from patterns.chapter_05_singleton.singleton import Singleton
+
+
+def test_singleton() -> None:
+    """Test signleton pattern.
+
+    Two created instances should be the same object.
+    """
+    s1 = Singleton()
+    s2 = Singleton()
+
+    assert str(s1) == str(s2)