Add end-to-end tests for reading+writing

[kevinzwang]

We need more tests that cover

• [ ] Writing and then reading data (consistency)
• [ ] Reading and then writing data (complete workflow)
• [ ] More tests for table formats on various clouds

Test matrix:

	S3	Azure	GCS
Iceberg reads	✅	✅	❌
Iceberg writes	❓	❌	❌
Iceberg upserts	❌	❌	❌
Delta Lake reads	❓	❌	❌
Delta Lake writes	❓	❌	❌
Delta Lake upserts	❌	❌	❌