> The directory with test files can be read-only.

test_regrtest skips tests which requires to write in Lib/test/ if the write fails.

        except PermissionError as exc:
            if not sysconfig.is_python_build():
                self.skipTest("cannot write %s: %s" % (path, exc))

> test_regrtest should use temporary directory.

Yes! That's the purpose of this issue!

But I was unable to find a technical solution to implement this. I need to try harder :-)