> Shall I commit the new file or first add more tests?

Might as well commit now; there's no sense in leaving the code sitting
around just because the file feels a bit short.

A couple of minor nits about your patch, though:

- The docstring for test_tools says "Tests for reindent.py". It would be
  better to have something referring to the Tools directory, so people
  know that this is the right place to put new tests for scripts that
  currently don't have any.

- The wording of the message 'requires an uninstalled Python build' is a
  bit awkward - I'd use "non-installed" instead of "uninstalled", so
  inexperienced users don't go and try to uninstall their system Python ;-)