[docs] [issue18578] Rename and document test.bytecode_helper as test.support.bytecode_helper

[Stéphane Wirtel]

Stéphane Wirtel <stephane at wirtel.be> added the comment:

Other discussion with Zach, and at the end, there is no problem to merge your PR.

Thank you for your contribution

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<https://bugs.python.org/issue18578>
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