Add function tests

test/config_test.dart

@@ -0,0 +1,13 @@
+import 'package:configurable/system_config.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('get system config by key', () {
+    var key = 'app.name';
+    var value = 'CUBETIQ';
+    var result = SystemConfig.getOrNull(key, defaultValue: value);
+
+    expect(value, equals(result));
+    expect(value, equals(SystemConfig.getOrNull(key)));
+  });
+}