gh-91916: Fix test_runpy on non-UTF-8 locale (GH-91920)

If use a non-builtin codec, partially implemented in Python
(e.g. ISO-8859-15), a new RecursionError (with empty error message)
can be raised while handle a RecursionError.

Testing for error message was needed to distinguish
a recursion error from arbitrary RuntimeError. After introducing
RecursionError, it became unnecessary.