[stdlib] Improve tests for `abs`, `divmod`, `round` free functions

[laszlokindrat]

These delegate to dunder methods, which should be tested separately for each structs that implement them. However, we do need a few basic tests to ensure that the free functions do work (e.g. so we don't accidentally rename them).

See comment: 65f0e5b#diff-2812447c50533870693b58783ccb2c24de6d8d3083372eb17084dbe369eb87a6R43-R45

[msaelices]

@laszlokindrat can I work on this?

[laszlokindrat]

@msaelices it's yours, thank you!

[msaelices]

PR ready for review: #2724

[msaelices]

Should we close this?

[laszlokindrat]

Yes, thanks for fixing it!