Provide a way for an admin to specify which tests to run

Problem/Motivation

Triggered by #2585741: server IP blacklisted the idea was suggested that some of the various tests that are done to verify the email address could be disabled.

Proposed resolution

Create a mechanism where a system admin can choose which tests to execute when checking the email addresses.

Remaining tasks

1. Fix the issue
2. Create a patch
3. Review the patch
4. Commit the patch

User interface changes

This will need some admin UI changes.

API changes

TBD.

Data model changes

TBD.