Added unittest for naturalLogarithmFunction

src/test/c/test_taschenrechner.c

@@ -138,5 +138,10 @@ void test_logarithmFunction(void) {
     TEST_ASSERT_EQUAL_FLOAT(1.0, logarithmFunction(10.0));
 }
 
+// Natural logarithm function
+void test_naturalLogarithmFunction(void) {
+    // Your test code here
+    TEST_ASSERT_EQUAL_FLOAT(0.0, naturalLogarithmFunction(1.0));
+}
 
 #endif // TEST