Add swift scheduling test + update perfect-numbers docs

[resu-xuniL]

following problem-specifications

swift scheduling got a new test + adjust example.php

(and update perfect-numbers docs)

[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.

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[mk-mxp]

Thanks for helping! Such a small new test case and so much to correct...

[resu-xuniL]

You're welcome!
I must admit that only one change would have been necessary for the new test (the round() replacement with intdiv()). But I have seized the occasion to remove useless code :)