(PUP-2912) Build/unpack spec tests w/o symlinks

Prior to this commit we would try to use symlinks in the tests on platforms that
do not support symlinking.
This commit uses the Puppet feature checker to conditionally try the tests if
Puppet thinks the platform supports symlinks (Unixes, Linuxes, Win 2k8+).
These are normally gated by `it "...", :if => Puppet.features.manages_symlinks?`. That we we get that it is skipping the test, I think.