Only pass -E to the child interpreter if our interpreter was running in that

Only pass -E to the child interpreter if our interpreter was running in that
mode.  Explicitly remove the PYTHONFAULTHANDLER environment variable before
launching a child interpreter when its presence would impact the test (the
reason -E was being used in the first place).

This enables running the test in an environment where other Python environment
variables must be set in order for things to run (such as using PYTHONHOME to
tell an embedded interpreter where it should think it lives).