gotta-snatch-em-all: change task to use symmetric set difference

[sanderploegsma]

Following the discussion in this forum topic, this PR updates the third task in the gotta-snatch-em-all exercise to remove some ambiguity.

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[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.

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[rkabhishek]

Hi, thanks for fixing this. Can anyone explain why we are not rerunning the tests for this exercise? This change affects one test.