Add comment, test to unescapeHTML (#7949)

app/javascript/mastodon/utils/__tests__/html-test.js

+import * as html from '../html';
+
+describe('html', () => {
+  describe('unsecapeHTML', () => {
+    it('returns unescaped HTML', () => {
+      const output = html.unescapeHTML('<p>lorem</p><p>ipsum</p><br>&lt;br&gt;');
+      expect(output).toEqual('lorem\n\nipsum\n<br>');
+    });
+  });
+});

app/javascript/mastodon/utils/html.js

+// NB: This function can still return unsafe HTML
 export const unescapeHTML = (html) => {
   const wrapper = document.createElement('div');
   wrapper.innerHTML = html.replace(/<br\s*\/?>/g, '\n').replace(/<\/p><p>/g, '\n\n').replace(/<[^>]*>/g, '');