Test for SchemaAgreementError::RequiredHostAbsent

https://github.com/scylladb/scylla-rust-driver/pull/1355 introduced this variant, but test was not written for it, since it is difficult.
See the discussion: https://github.com/scylladb/scylla-rust-driver/pull/1355#discussion_r2107589683