[meta] Test Runner Re-Architecting

Problem/Motivation

The test runner needs some re-architecting in order to maximize maintainability while adding flexibility for new test features.

This will be a conservative process, beginning with functional tests in order to have a baseline behavior.

Next, we will re-work the way configuration is loaded.

After that, we will do more things, and they will be listed here. :-)

Proposed resolution

In this order:

1. #2680565: [meta] Create functional tests of run command output to test production essentials
2. #2809941: Re-think config
3. #2694577: Use build definition files instead of configset files

Remaining tasks

User interface changes

API changes

Data model changes