(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