Adds the hidden_field_selector
I'm in the process of converting my project to DTT and hit the following error from an existing test:
InvalidArgumentException: Selector "hidden_field_selector" is not registered.
This comes from calling \Drupal\Tests\WebAssert::hiddenFieldExists
.
The hidden_field_selector is registered in \Drupal\Tests\BrowserTestBase::initMink
so we just need to do the same.