(PUP-2732) Restrict Posix exec tests to POSIX env

 - Instead of trying to simulate a POSIX environment on the Windows for
   the sake of tests against a provider that never runs on Windows,
   exclude the tests when Puppet.features.posix? is not true.
 - Given that these tests will no longer execute on Windows, the usage
   of expand_path is no longer necessary, nor are any of the
   Puppet.features.microsoft_windows? guards