tests.script_helper provides various utilities for sensibly testing scripts in a subprocess. It isn't easy for CPython developers to discover, since it is isn't documented and the file is mixed in with actual tests in the main test directory.

As discussed in #15494, it should be:

1. Moved from Lib/test/ to Lib/test/support (and imports in tests adjusted accordingly)
2. Documented in Doc/library/test.rst

These changes should be made on both the 3.3 and default branches to avoid spurious merge conflicts.