paasio: simplify tests

[senekor]

[no important files changed]

part of #1824

This is by no means perfect, but I attempted to make the test functions a little more concise and readable. This is desirable when the test code is shown to users online, while it wasn't a requirement when these tests were generated by macros.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

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[no important files changed]

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