cleanup Endurance structure

Similar to #806, the code for this test should be a library in `Sources/`. The main program should be a small main function, and then we can add an integration test (and unit tests if needed) in `Tests/`.

Start by inspecting the Endurance test code and compare its layout with the approach requested in issue #806. Move reusable code into Sources/, keep the main program as a small entry point, and add the integration test in Tests/; add unit tests only if needed, then run the Swift test suite.

Written by the indexing model from the issue text.