s3tests: add tests for per-bucket cloud transition targeting

s3tests: add tests for per-bucket cloud transition targeting

Add tests to validate the target_by_bucket feature which allows each
source bucket to transition objects to a dedicated destination bucket
rather than sharing a common target.

New tests:
- test_lifecycle_cloud_transition_target_by_bucket: validates objects
  land in bucket-specific targets without source bucket name prefix
- test_lifecycle_cloud_transition_target_by_bucket_multiple_buckets:
  validates isolation between different source buckets

Each test will perform a restore of the object transitioned.

Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>