Fixes#1643
`GetBucketLocation` in S3 returns empty `LocationConstraint` if the bucket is in the `us-east-1` region. This fix returns empty `LocationConstraint` if the gateway region is `us-east-1`.
All the integration tests used to be in a single file, which had become large, messy, and difficult to maintain. These changes split `tests.go` into multiple files, organized by logical test groups.