Make all testcase files available in the sandbox

[jorg-vr]

Users would expect files that are available to the judge, to also be available when developing a solution in the sandbox.

Papyros already has features to provide files since #5336
The change would mostly be getting the information from the judge which files should be present.

A complication might be that the judge is able to run all tests in isolation. Providing all files at once in the sandbox might have undesired effects.
We should also analyze how large these files can become...

This issue was created after an email for a teacher, who expected this behavior to already exist.