[AdminToolbarSearchSettingsForm] Add basic automated Functional Tests

Quick follow-up on this issue:

Copied over the same Tests class from the admin_toolbar_tools module and made a few adjustments to match with the admin_toolbar_search module.

This is a very basic initial Tests class, with the bare minimum:

• Browse to admin/config/user-interface/admin-toolbar-search-settings
• Submit the form
• Check the settings were saved with a success message.

Since all the jobs and tests were still passing 🟢, I went ahead and merged the changes above at #4.

This class should allow us to detect any issues with the schema or any other form related code and be used as the basis for adding more checks and automated tests for the admin_toolbar_search module.

Marking issue as Fixed for now.

Thanks in advance!

Automatically closed - issue fixed for 2 weeks with no activity.