Allow GridFields without titles to be found.

This fixes an issue with a silverstripe-cms behat test (insert-an-image.feature). In some cases, a GridField won't have a title element, which means that getGridFieldTable() won't be able to find it. This uses the data-name attribute, which is added to the <fieldset> that surrounds a GridField.

It doesn't appear that we can just use one method over another, as data-name won't exist unless there is no visible title.