add test for editor saga

src/sagas/editor.test.ts

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2020 The Pybricks Authors
+
+import { Ace } from 'ace-builds';
+import { mock } from 'jest-mock-extended';
+import { AsyncSaga } from '../../test';
+import { reloadProgram } from '../actions/editor';
+import editor from './editor';
+
+test('reloadProgram', async () => {
+    const saga = new AsyncSaga(editor);
+
+    const mockEditor = mock<Ace.EditSession>();
+
+    saga.setState({ editor: { current: mockEditor } });
+    saga.put(reloadProgram());
+
+    expect(mockEditor.setValue).toHaveBeenCalled();
+
+    await saga.end();
+});