[issue11978] Report correct coverage.py data for tests that invoke subprocesses

[Nick Coghlan]

Nick Coghlan added the comment:

To be more specific regarding `sitecustomize.py`:

    $ echo "print('Hello from sitecustomize.py')" > Lib/sitecustomize.py
    $ ./python -c "print('Hello from command line')"
    Hello from sitecustomize.py
    Hello from command line

Since we only need these instructions to work for a local checkout, we can rely on the `sitecustomize.py` hook.

It means we'll still miss coverage results from subprocess tests run in isolated mode or with site.py processing disabled, but those are both relatively rare and involve *not* running code that is normally run, so shouldn't impact the aggregate coverage results.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11978>
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