s3/test: refactor retention tests (#1615)
s3/test: refactor retention tests (#1615)

Coalesce retention period tests. This has the following effects:
- serializes the tests
- reuses the bucket and reduces bucket creation calls

Also:
- check whether bucket exists before attempting to create it
- nit: rename test for clarity and rename non-AWS specific tests
- nit: cleanup test logging and contexts

kopia/kopia,kopia/kopia,kopia/kopia,kopia/kopia