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Use a default FS mock for all tests
Do not duplicate setup code

This prevents us from taking shortcuts with the file creation time, and
will prove itself helpfull when we need even more informations about
file

Require a simple absolute file path to call createFile.
This is more coherent with the fact that IDirectory always returns an
absolute path when calling files() and dirs()

This is mostly aiming at allowing us to fiddle with the mock FS during
tests.

This is more convenient to set a FK to null when it's unset, rather than
having to write 2 different insert statements

This also ensures we are using the absolute path instead of the user
provided one