It's ugly and will cause a window to flicker, but the alternative
requires either ugly patches in VLC, or to encode a memory snapshot to
jpeg & save it to file ourselves

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()