Add empty persistence test
Simple persistence test
Refactor: add parameter to BasicTestCase.__openFile to specify sub-folder to write or save test data
Store consts of test data usage in TestDataUsage class
Refactor: extract method for getting file name of test data file
Add Persistence.testSave
Add Persistence.testLoadDead
Add Persistence.testLoadDeadAndRevive, Persistence.testGithubLoad
Add Persistence.testLoadAndUpdate. Bug: self._url missing
Fix CompletableGithubObject._url persistence
Clean up