Third party code can use tests.script_helper.

I prefer to do these changes in several steps:

1) Move script_helper.py to Lib/test/support/, document it, and create an alias (with deprecation warning).
2) Ensure that this doesn't break any buildbot.
3) Change tests to use test.support.script_helper instead of test.script_helper.
4) One or two versions later remove an alias.