[docs] [issue18576] Document test.support.script_helper

[Christie]

Christie added the comment:

@ncoghlan I've taken the test.rst changes from the previous diff and reviewed them against the latest script_helper (including updates from issue24033). Nothing had fallen behind, but I've added what was missing.

* I've realized that spawn_python and the helper run_python I added in issue24033 are basically the same so I will reconcile those
* Is there some way to load the docstrings from script_helper.py in test.rst instead of writing explanations for each method twice?

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Added file: http://bugs.python.org/file39645/issue18576.patch

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