Update strain tests

[BNAndras]

I added the tests toml file, two missing canonical tests for lists, and fixed a minor typo. I suspect previously passing solutions would most likely pass these two tests so this might be a case for not rerunning the solutions (your call though obviously).

[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.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

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