Ignore tests related to fdbsql

I've written an adapter for FoundationDB's Sql layer: https://github.com/FoundationDB/sql-layer-adapter-sequel Which runs the Sequel integration tests as part of it's testing suite. This minor change ignores the handful of tests that were failing, due to unsupported features in the Sql layer. I couldn't figure out a way to mark the `Sequel::Plugins::ConstraintValidations` example group pending, so that on is just in an if block.