(#7656) Port PMT unit test into Puppet core

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

This patch ports the PMT unit tests into core Puppet with the following
major changes and refactorings.

All spec tests are being updated to require modules located in their new
location in the core Puppet codebase: lib/puppet/module_tool.

The following tests are being removed:

• spec/unit/application_spec.rb

application_spec.rb is being removed because the tests are no longer
valid, all examples are related to custom settings code that is not
being ported over.

The following tests are being renamed:

• spec/unit/tool_spec.rb
  
  tool_spec.rb is being renamed to module_tool_spec.rb. This change is being made in order to match the new name of the module under test: module_tool.rb

The following tests are being relocated:

• spec/unit/repository_spec.rb
• spec/unit/semver_spec.rb

Both repository_spec.rb and semver_spec.rb are being relocated under
the spec/unit/module_tool directory in core Puppet. This change is
being made to match the new locations of the modules under test.