The scripts/run-tests.sh script has matured a lot during D8, and become the preferred way to run tests (docs). It's main advantage is that there can be no leakage between the parent Drupal site and child Drupal site (because there is no child site when --sqlite is used). This vastly simplifies debugging test failures, and vastly simplifies the test runner itself. However, many folks still appreciate having a UI for choosing tests to run.
This issue proposes to change what happens when the simpletest form is submitted. Instead of actually running the tests, we print out a bash command which calls run-tests.sh with the parameters for chosen by the user in the GUI. More importantly, we can finally rip out all of the code in Drupal which deals with isolation/leakage. We are one step closer to using phpunit as our test runner.