[3.6] bpo-33861: Minor improvements of tests for os.path. (GH-7715) by miss-islington · Pull Request #7752 · python/cpython

@serhiy-storchaka @miss-islington

* Test exists(), lexists(), isdir(), isfile(), islink(), ismount()
  with bytes paths.
* Remove unneeded silencing DeprecationWarning for ismount() with
  bytes path.
* Test common functions with unencodable and undecodable paths.
* Minor clean up and refactoring.
(cherry picked from commit 17a0088)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>