Behat mod_dataform support for field default content test.

Behat mod_dataform support for field default content test:

• New method behat_mod_dataform::scenario_field_default_content(). This methods returns a list of steps for the test. It creates a dataform and a default view, and then collates from all installed fields the field specific steps for testing the field default content.
• New behat feature test: Field default content. This feature test contains one scenario that runs the above method.