(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.)

What if the schema is invalid? That's my only concern. Not regularly testing it as part of specs. I didn't realize this would be a platform specific thing. Does this mean that the use of it is also platform specific? I'm guessing no on that.