For the integration tests we prefer non-linked test cases because those
PDF files are directly checked into the Git repository and thus don't
need a separate download step that linked test cases do.
However, for the freetext and ink integration tests we currently use
`aboutstacks.pdf` which is a linked test case. Fortunately we don't need
to use it because for most tests we don't actually use any properties of
it: we only create editors on top of the canvas, but for that any PDF
file works, so we can simply use the non-linked `empty.pdf` file instead.
The only exception is the "aria-owns" test that needs a line of text from
the PDF file, so we move that particular test to a dedicated `describe`
block and adapt it to use the non-linked `attachment.pdf` file that just
contains a single line of text that can be used for this purpose.
The changes combined make 12 more integration tests run out-of-the-box
after a Git clone, which also simplifies running on GitHub Actions.
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