atbash-cipher: Sync tests with problem specifications

[tasxatzial]

Last two decode tests have been marked as not implemented because the inputs are not valid encoded strings. They should be removed from the specs as well.

https://forum.exercism.org/t/atbash-cipher-are-the-last-two-tests-invalid/13482

Also, this update will invalidate all solutions, as the old tests did not even include the decoding part of the exercise.

[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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