[backend] new s3 test cases

[alxndrsn]

Ref https://forum.getodk.org/t/s3-bucket-migration-for-central-storage/58029

Add test cases to document the following S3 integration implementation details:

• that object path doesn't depend on bucket name or other parts of the provider URL
• what happens if s3 is down (no response, 5xx response etc.) to:
  • uploading objects, and (test/e2e/s3: add tests for S3 server 5xx central-backend#1825)
  • retrieving objects (test/e2e/s3: add tests for S3 server 5xx central-backend#1825)