bpo-12383: Refactor subprocess test_empty_env (#1874)

Bugfix: This test wasn't being run because it was skipping based on the
presence of Py_ENABLE_SHARED rather than its value.  It is always present
on POSIX systems but defaults to 0.

Refactoring: Move the environment variables that can be ignored into a
function.  Parse the list from the child process and filter out the ones
to exclude in the parent before checking that the rest is empty.

Feature: Adds always present environment variables to ignore when
running in a Gentoo sandbox so that the test can pass there.