(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.
I could definitely add it to specs. I debated that since it's not strictly part of the lib code that's being spec tested. But I could add a test that the *schema* is valid to, say, `specs/schema`. (The test that the *facts* are valid is platform-specific, which is why there's the acceptance test.)

Thoughts?

(And sorry, I didn't get the "normally" comment two up.)