Fixes#1379
Adds validation for bucket canned ACLs in `CreateBucket` and `PutBucketAcl`. The gateway supports three values: `private`, `public-read`, and `public-read-write`. All other values (including `authenticated-read`, which is not supported) are considered invalid and result in an `InvalidArgument` error with an empty error message.
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.
