(#7656) Port PMT test fixtures into Puppet core

Before this patch, the Puppet Module Tool (PMT) test fixtures lived in
the PMT codebase -- in a separate project.

This patch ports the PMT test fixtures into core Puppet. There are no
major changes or refactorings. This patch places the PMT test fixtures
under the spec/fixtures directory; the same relative location of the
previous codebase.