Storage Fixtures framework

[qc00]

(Separated out from #473)

Separated the test fixture setup logic from conftest.py to its own framework:

• Proper documented API
• Callable without pytest
• Full life-cycle management with proper shutdown and clean-up
• Can have multiple isolated fixtures open at the same time
• Permission control feature to support testing of Warn if no credentials are in effect for S3 #473 and Use specific exception type for certain kinds of storage failures #447