Issue #12383: fix test_empty_env() of subprocess on Mac OS X
Issue #12383: fix test_empty_env() of subprocess on Mac OS X

Mac OS X adds __CF_USER_TEXT_ENCODING variable to an empty environment. Fix
also the test on the Py_ENABLE_SHARED config varible: test that the variable is
present, don't check it's value.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator