Expand tests for Spark external tables

(split off from #10)

### Not currently tested
- [ ] ndjson (`s3://dbt-external-tables-testing/json/`)
- [ ] Hive-formatted external tables, since SQL Endpoint returns this error:
```
('42000', '[42000] [Simba][Hardy] (80) Syntax or semantic analysis error thrown in server while executing query. Error message from server: Error running query: org.apache.spark.sql.AnalysisException: Cannot use this data source. Only csv,json,avro,delta,parquet,orc,text data sources are supported on SQL Gateway.;;\nSubqueryAlias spark_catalog.dbt_jcohen.people_csv_unpartitioned_hive_format\n+- HiveTableRelation `dbt_jcohen`.`people_csv_unpartitioned_hive_format`, org.apache.hadoop.hive.serde2.lazy.LazySimpleSerDe, [id#2612, f\x00 (80) (SQLExecDirectW)')
```