test: adjust schema test for broader use cases

src/resources/foo.ts

@@ -14,6 +14,9 @@
  */
 
 /**
+ * This is a simple interface declaration for
+ * testing the schema generation and validation.
+ * 
  * @validatable
  */
 export interface Foo {
@@ -21,4 +24,15 @@ export interface Foo {
    * Dummy parameter
    */
   lorem: 'ipsum';
+
+  /**
+   * String literal type property
+   */
+  type: FooType;
 }
+
+/**
+ * This is a simple type declaration for
+ * usage in the Foo interace.
+ */
+export type FooType = 'Foo';