In line with the other complex JSON output formats for plan and provider
schema, here we add an explicit `format_version` field to the JSON
output of terraform validate.
Also uncomment and fix some tests which had been skipped for a couple of
years. Those validate cases work now!
Note that these test cases and the JSON output are not especially
minimized, making them snapshot/golden tests. The output looks correct
at time of writing, and we don't expect to change validate significantly
any time soon, but if we do there will be some churn here.