Skip some tests that require a subinterpreter launched with -E or -I when the

Skip some tests that require a subinterpreter launched with -E or -I when the
interpreter under test is being run in an environment that requires the use of
environment variables such as PYTHONHOME in order to function at all.

Adds a private test.script_helper._interpreter_requires_environment() function
to be used with @unittest.skipIf on stdlib test methods requiring this.