Currently adding tags is only possible by inserting them into test title, e.g.:

test('Test login page @fast @mobile', async ({ page }) => {
  // ...
});

It may have some downsides for reporting when you get very long titles, see #17375

Suggestion is to introduce test.tags() method that will add tags to current test without altering the title:

test.tags('@fast', '@mobile');
test('Test login page', async ({ page }) => {
  // ...
});