tests: Add a comment explaining why `Promise.all()` is not used

src/tests/frontend/helper.js

@@ -37,6 +37,8 @@ const helper = {};
     };
 
     if (!win.$) await load('../../static/js/vendors/jquery.js');
+    // sendkeys.js depends on jQuery, so it cannot be loaded until jQuery has finished loading. (In
+    // other words, do not load both jQuery and sendkeys inside a Promise.all() call.)
     if (!win.bililiteRange && includeSendkeys) await load('../tests/frontend/lib/sendkeys.js');
 
     win.$.window = win;