Test timeout issues

[mk-mxp]

PHPUnit 10.5 assertArrayNotHasKey()

PHPUnit ^10.3 has an issue with assertArrayNotHasKey(). Only this (and the opposite assertArrayHasKey()) are affected. The issue is not listed in PHPUnit's issue list / mentioned in Pull Requests, but the issue is gone in 11.0.0.

This contains a workaround for this and enables upgrading to PHPUnit 10.5. I replaced that assertion with a functionally equivalent one, but that prints some misleading output in addition to the helpful message. So it should be reverted when upgrading to PHPUnit 11.

CI limit for test runtime

I also added a time limit of 18 (lowest value possible on my machine with equal performance to Online Test Runner) x 3 (to compensate lower CI performance) = 54 seconds for each exercise test class using timeout command. This should prevent running into performance degradation issues in the future.

As discussed in the forum there is no recommendation or guiding data for such a limit. Only guesses about "test runner is expected to be slower than GitHub CI". But the production limit of 20 seconds exists and is enforced. So I took a guess based on my measurements.

Part of #652

[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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[mk-mxp]

@homersimpsons Sorry, I thought I had figured that out right. But I somehow haven't remembered things correctly...

[mk-mxp]

@homersimpsons Another attempt using measured data now. I put the data into the forum, and a longish comment into the test script.

[homersimpsons]

Thank you for your investigation. I'm proposing a new wording for the comment about the timeout. But I still approve the pull request as-is.