Enhance and modernize test_os

Enhance and modernize test_os

* add create_file() helper function
* create files using "x" mode instead of "w" to detect when a previous test
  forget to remove a file
* open file for writing in unbuferred mode (buffering=0)
* replace "try/finally: unlink" with self.addCleanup(support.unlink)
* register unlink cleanup function *before* creating new files