Add tests for optional fields

tests/samples/optionals/source.js

@@ -0,0 +1,9 @@
+type Type = {
+    a?: string,
+};
+
+interface Interface {
+    a?: boolean;
+};
+
+export {Type, Interface};

tests/samples/optionals/types.yaml

@@ -0,0 +1,12 @@
+-   kind: record
+    fields:
+    -   name: a
+        value: {kind: string}
+        required: false
+    id: [optionals, Type]
+-   kind: record
+    fields:
+    -   name: a
+        value: {kind: boolean}
+        required: false
+    id: [optionals, Interface]