I went for something even simpler: one new file Lib/test/test_tools.py.  There is no need to have a file there and more files in Tools/tests, everything can go in test_tools (I’m not worried about file size, we already have a few thousand-liners test modules).  I think it’s the simplest thing that works.

My only concern is communication: how do we tell people working on a tool that they should write a test in test_tools?  I’m not sure they would read Tools/README; maybe a note at the top of the Python files would work; a note in the devguide; sending an email to python-dev to tell about the new file + reminders when we see a new tool bug opened.