Move SimpleConregAdminMemberAddOns into Form directory

Problem/Motivation

Forms were originally created in the src directory, because I knew nothing. I've been moving them into better locations and this is continuing that work. This particular form shows a report of the members who have selected options on the registration form.

This is the last form still in the src/ directory, so moving this out is a minor milestone in the project.

Steps to reproduce

N/A

Proposed resolution

Because this form is part of the admin interface, it should be moved to the src/Form/Admin directory. It should also have a more descriptive class name, and be added to the build tests.

Remaining tasks

• Move SimpleConregAdminMemberAddOnsinto src/Form/Admin.
• Rename filename and class to "MemberAddOns".
• Update conreg.routing.yml with the new form name.
• In tests/src/Kernel/FormBuildTest.php, copy one of the existing test cases. Update the test function name, the route name, and make sure the default title match the one from the routing file, and the form ID check matches the form ID.
• Verify new test passes.

User interface changes

None.

API changes

None.

Data model changes

None.