run-tests: move unicode-to-bytes operations on paths to a helper (issue4667) This doesn't fix the probably-wrong utf-8 encoding choice, it just starts the process of encapsulating all the path handling in run-tests in a single place. One known-path use of .encode() remains: it's related to use of mkdtemp, and it will be fixed in a followup patch once we have a companion _strpath() helper function to go from bytes to a str, as we need to file a bug about mkdtemp upstream.