* 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 17a0088e)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>