[3.12] gh-76785: Make test.test_interpreters executable (GH-112982) (GH-113470)

This is so that we can run python -m test.test_interpreters. As such it
backports that aspect of commit 86a77f4e1a5ceaff1036b0072521e12752b5df47,
where it is implemented by a package __main__.py.