Make get_dir_creation_time use files creation dates, not folder

Description

We cannot rely on the directory nor file creation date, as it might get modified with filesystem upgrades, as was done recently in January.
File modification date are conserved though, so we must use these when the information is not available from run metadata.

How Has This Been Tested?

The unit test for it was updated.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@hammerd @ahmedk
@tmichela this is the type of stuff you like