(FM-7388) Fix load_module_metadata_spec for Puppet 6.0.0
(FM-7388) Fix load_module_metadata_spec for Puppet 6.0.0

Before this the test would fail because it used `subject.call` which
hits a compatibility method in rspec_puppet that expects to be called
with `scope`.

This is now fixed by instead using `subject.execute`
puppetlabs/puppetlabs-stdlib,DavidS/puppetlabs-stdlib