(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
Merge pull request #2884 from Iristyle/ticket/master/PUP-2732-fix-POSIX-exec-specs-for-Windows

(PUP-2732) Restrict Posix exec tests to POSIX env
(PUP-2882) Fix single available upgrade

Prior to this commit `puppet module upgrade` would fail with an error message
about not being able to satisfy the upgrade when there is only one published
release of the module.
This commit changes the error message to say that there is no version to upgrade
to. This should reduce some user confusion.