Test PDF files should never be executable because we only read their
contents, so this commit makes sure that all test PDFs have the same
permissions, namely 0644 (read-only for all groups, and writable for the
owner), to limit their permissions for a least-privilege approach.
When a pdf as a FreeText without appearance, we use a fake font in order to render it
and that leads to create few new refs for the font.
But then when we're saving, we create some new refs which start at the same number
as the previous created ones.
Consequently, when saving we're using some wrong objects (like a font) to check if
we're able to render the newly added FreeText.
In order to fix this bug, we just remove the persistent refs (which are only used
when rendering/printing) during the saving.
