Make test_compileall more robust by using -S to keep sys.path minimized.

Make test_compileall more robust by using -S to keep sys.path minimized.

Try this again, hopefully the right way this time.

Arfrever Taifersar Arahesis reported that test_compileall failed during Gentoo
install because it was tyring to write .pyc files to a read-only system
directory during test_no_args_compiles_path.  Having the tests call python
with -S should eliminate the system directories from the path.