Implement HTML to Markdown integration tests

spec/frontend/vue_shared/components/rich_content_editor/rich_content_editor_integration_spec.js

+import Editor from '@toast-ui/editor';
+import { registerHTMLToMarkdownRenderer } from '~/vue_shared/components/rich_content_editor/services/editor_service';
+import '@toast-ui/editor/dist/toastui-editor.css';
+
+describe('vue_shared/components/rich_content_editor', () => {
+  let editor;
+
+  const buildEditor = () => {
+    editor = new Editor({
+      el: document.body,
+    });
+
+    registerHTMLToMarkdownRenderer(editor);
+  };
+
+  beforeEach(() => {
+    buildEditor();
+  });
+
+  describe('HTML to Markdown', () => {
+
+    it('uses "-" character list marker in unordered lists', () => {
+      editor.setHtml('<ul><li>List item 1</li><li>List item 2</li></ul>');
+
+      const markdown = editor.getMarkdown();
+
+      expect(markdown).toBe('- List item 1\n- List item 2');
+    });
+
+    it('does not increment the list marker in ordered lists', () => {
+      editor.setHtml('<ol><li>List item 1</li><li>List item 2</li></ol>');
+
+      const markdown = editor.getMarkdown();
+
+      expect(markdown).toBe('1. List item 1\n1. List item 2');
+    });
+
+    it('indents lists using four spaces', () => {
+      editor.setHtml('<ul><li>List item 1</li><ul><li>List item 2</li></ul></ul>');
+
+      const markdown = editor.getMarkdown();
+
+      expect(markdown).toBe('- List item 1\n    - List item 2');
+    });
+
+    it('uses * for strong and _ for emphasis text', () => {
+      editor.setHtml('<strong>strong text</strong><i>emphasis text</i>');
+
+      const markdown = editor.getMarkdown();
+
+      expect(markdown).toBe('**strong text**_emphasis text_');
+    });
+  });
+});