Sync simple linked lists

[kahgoh]

This updates the exercise to sync the canonical test data. Note, this change also attempts to maintain support for solutions written before the canonical data was added. The test differs from the canonical data in the following ways:

• len property is used in place of a dedicated count() function
• to_array is used in place of to_list
• to_array is used for validation in the reverse tests

[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]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!