gh-76785: Fix Windows Refleak in test_interpreters (gh-117913)

gh-117662 introduced some refleaks, or, rather, exposed some existing refleaks.  The leaks are coming when test.support.os_helper is imported in a "legacy" interpreter.  I've updated test.test_interpreters.utils to avoid importing os_helper, which fixes the leaks.  I'll address the root cause separately.