Problem/Motivation
Based on conversation in #3057420: [meta] How to deprecate Simpletest with minimal disruption, it's likely that we'll move simpletest module out of core, to be a contrib module.
This will allow some flexibility for those who wish to use simpletest's WebTestBase framework, and/or also use the UI test runner.
This issue is filed against 8.8.x, but it is much more likely to happen in 8.9.x or 9.0.x.
Proposed resolution
Move simpletest module out of core to be a contrib module.
Remaining tasks
How to deprecate and remove the module while not ruining people's day?
run-tests.sh (at least for now) will run simpletest tests if simpletest module is installed. Can DrupalCI automatically install simpletest module or does it need to be an explicit dependency? If it's an explicit dependency can we prompt somehow?
The contrib module will need manual testing for the simpletest UI and run-tests.sh to ensure they actually work as part of making a stable release.