Adjusted Config tests for proper singleton pattern

src/test/java/de/rwu/easydrop/util/ConfigTest.java

@@ -2,6 +2,7 @@ package de.rwu.easydrop.util;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.mockito.Mockito.spy;
 
 import javax.naming.ConfigurationException;
@@ -20,17 +21,18 @@ class ConfigTest {
     }
 
     @Test
-    void testGetInstanceNull() {
-        config = null;
-
+    void testGetInstance() {
         Config newConfig = Config.getInstance();
         assertNotNull(newConfig);
     }
 
     @Test
-    void testGetInstanceNotNull() {
-        Config newConfig = Config.getInstance();
-        assertNotNull(newConfig);
+    void testGetInstanceEquality() {
+        // Create "two" instances to check validity of Singleton pattern
+        Config instance1 = Config.getInstance();
+        Config instance2 = Config.getInstance();
+
+        assertSame(instance1, instance2, "Instances should be equal");
     }
 
     @Test