Change Simpletest UI from a test runner to a CLI snippet generator

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.