Use pytest temporary folders and fixtures to create file hierarchy at test time

[andrii-i]

Improves Jupyter Scheduler pytest testing suite:

• Provide root and staging folders as fixtures using temporary folders instead of hard-coding paths in tests and storing folders in the codebase
• Store all test files in static folder, create file hierarchy to be tested at test time using temp folders and fixtures
• Make test db fixture return a session so that there is no need to instantiate a session every time

Fixes #514.