tests: remove static data from presentation

While get_presentation() will only ever be called once (making the caching of
the return value moot), it is good to stop using the static variable as it
would cause surprising problems if anyone adds more tests here and runs it
under the new test harness.

It was leaked before and continues to be so.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
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