gh-54781: Move Lib/lib2to3/tests/ to Lib/test/test_lib2to3/ (#94049)

* Move Lib/lib2to3/tests/ to Lib/test/test_lib2to3/.
* Remove Lib/test/test_lib2to3.py.
* Update imports.
* all_project_files(): use different paths and sort files
  to make the tests more reproducible.
* Update references to tests.