[docs] Rename and document test.script_helper as test.support.script_helper (issue 18576)

[vadmium+py at gmail.com]

https://bugs.python.org/review/18576/diff/15165/Doc/library/test.rst
File Doc/library/test.rst (right):

https://bugs.python.org/review/18576/diff/15165/Doc/library/test.rst#newcode652
Doc/library/test.rst:652: variables *env_vars*, asserting that run
succeeds (``rc == 0``).
(return code is zero) might be better, to match the term used in the
next paragraph

https://bugs.python.org/review/18576/diff/15165/Doc/library/test.rst#newcode655
Doc/library/test.rst:655: appropriate :exc:`AssertionError`` for a
non-zero return code containing
Single back-tick at end of :exc:`AssertionError`

https://bugs.python.org/review/18576/diff/15165/Doc/library/test.rst#newcode658
Doc/library/test.rst:658: Trailing whitespace will be stripped from both
stdout and stderr.
I believe it’s only the stderr. See the strip_python_stderr() call.

https://bugs.python.org/review/18576/diff/15165/Doc/library/test.rst#newcode671
Doc/library/test.rst:671: variables, asserting that *env_vars* fails
(``rc != 0``).
asserting that the run fails (return code is nonzero), [similar to
above]

https://bugs.python.org/review/18576/diff/15165/Doc/library/test.rst#newcode786
Doc/library/test.rst:786: Returns True if our `sys.executable
interpreter` requires environment
Maybe this should be something like “:data:`sys.executable` interpreter”

https://bugs.python.org/review/18576/