search_embedded_form test module is improperly using config instead of state

Follow-up to #2105609: Convert search_embedded_form_form to a Controller

There's a module core/modules/search/test/modules/search_embedded_form that is used in tests.

This module has a conceptual problems that should be fixed: It is using the config system to keep track of how many times the form has been submitted. I think it should be using the state system. This will need to be changed in both this module and the SearchEmbedFormTest class that uses it.

Basically, calls to ....->config('search_embedded_form.settings')-get('submitted') (and the set method) need to be changed to .....->state('search_embedded_form.submit_count')->get() (or set()). And the config YML file also needs to be removed.

This seems like an excellent Novice project.