Convert \Drupal\field\Tests\NestedFormTest to BrowserTestBase

I created a partial conversion and am in doubt about it.

While converting I saw that the JS part of this test is testing drag-n-drop. There are many tests that test this already, in particular #2809483: Convert AJAX part of \Drupal\field\Tests\FormTest::testFieldFormJSAddMore to JavascriptTestBase.

The conversion can still be usefull as the test is a little less hacky than the WebTest but for now no JS is tested.

To be decided:
Should this test contain validation for the drag-n-drop?

If not:
- Should the current js-add-more test be extended to use multiple draggable fields?
- Should the current conversion be committed or only a patch stripping the Add more and draggable test part?

Thanks for answering my question.

I'll add the drag-n-drop functionality and update the issue with a new patch.

I updated the test to be an exact copy of the original, testing the dragging and add more behavrior. As a bonus the expanding of the second entity is also tested, this was required because the dragging and add more functionality couldn't be tested otherwise..

Update to conform to the new wait API.

I think it's better to convert whole NestedFormTest with minimal changes to BTB.
there is no interdiff because it's completely a new patch.

one thing to highlight:
instead of this

$this->assertFieldByName('entity_2[changed]', 0, 'Entity 2: changed value appears correctly in the form.');

I use this

$assert_session->hiddenFieldValueEquals('entity_2[changed]', REQUEST_TIME);

I don't know how it worked before, changed field will be the same as created after first save like this:

$storage = $this->container->get('entity_type.manager')
  ->getStorage('entity_test_constraints');

$entity_1 = $storage->create();
$entity_1->save();

here is a proof: \Drupal\Core\Field\Plugin\Field\FieldType\ChangedItem::preSave()

The use of AJAX here was as a workaround for limitations in WebTestBases drupalPostForm and we have proper coverage for the 'add more' javascript in \Drupal\Tests\field\FunctionalJavascript\FormJSAddMoreTest::testFieldFormJsAddMore. This seems sufficient for this, I think.

So just converting this to BrowserTestBase makes sense here I think.

The conversion looks good, nice and minimal.

Updated the IS to reflect the new direction.

Can someone update the issue summary - I've updated the title but now the issue summary is out-of-date. Once the issue summary is updated it can be set back to rtbc.

Updated summary.

Committed and pushed c93ced1ea2 to 8.7.x and 9082b8de9b to 8.6.x. Thanks!