(maint) Add validation of the facter schema

Previously we've had validation that the facts output by facter
conform to the facter schema, but no catch if the schema itself
was invalid.

This commit adds that additional validation step to both the
acceptance test and the one-off validate.rb script.
This is just a utility script for developers so that you can try out fact/schema changes on target machines easily.

I used `bundle exec` because it's so common, e.g. https://github.com/puppetlabs/puppet/blob/master/util/dev-puppet-master

But it's never run as part of specs/acceptance.