Fix #9650 - S3 tests failing

The S3-tests extend from Test\Files\Storage\Storage. Some methods in this class test way too much functionality at once (especially testStat), so I split some of the tests into smaller components. I realize changing core tests might be a touchy subject so please review the changes.

Most of the failing S3-tests regarded renaming/overwriting which was fixed by removing some unnecessary return calls. Two tests (both split from testStat) are still failing:
```
1) Test\Files\Storage\AmazonS3::testRootHasUpdatedAfterCreatingFile
Failed asserting that false is true.

/Users/johan/Desktop/OC/tests/lib/files/storage/storage.php:248

2) Test\Files\Storage\AmazonS3::testRootHasUpdatedAfterUnlinkingFile
Failed asserting that false is true.

/Users/johan/Desktop/OC/tests/lib/files/storage/storage.php:257
```
These fail because there is no reliable way (to my knowledge) to find out when a bucket was last modified.