isbn-verifier: Sync tests

[tasxatzial]

Also

• Updated the isbn_verifier.clj starter file with a proper function template
• Written a new example solution that passes the tests. It is expected that some of the community solutions will be invalided.
• Switch to idiomatic checking (= true x) -> (true? x)

[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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