Remove datalist_options.html.

This feature is better covered by
html/semantics/forms/the-datalist-element/datalistoptions.html.
Remove fieldset_form.html.

This feature is better covered by
html/semantics/forms/the-fieldset-element/HTMLFieldSetElement.html.
Write a test for the form IDL attribute and remove older tests for the same.
Merge pull request #1472 from Ms2ger/iw-forms

Remove some obsoleted Infraware forms tests.