high scores: Add testing to make sure methods don't mutate the scores list

[andrewthetechie]

from a conversation in slack: https://exercism-team.slack.com/archives/CASLGCCSZ/p1564786254008900

This is a common issue we are seeing as python track mentors. This should catch students using pop to return values and doing sorts or reverse rather than sorted() or reverse()

[yawpitch]

For anyone that comes across this we are currently considering mutation acceptable in the function-based high-scores exercise. There is a PR to update the mentor notes to reflect this here and also see prior discussion in #1868, #1745, and #1735.