Closes#1714
There is a `Location` field in the `CompleteMultipartUpload` result that represents the newly created object URL. This PR adds this property to the `CompleteMultipartUpload` response, generating it dynamically in either host-style or path-style format, depending on the gateway configuration.
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.
