Improve test coverage

[BooleanCat]

Please provide a brief description of the change.

Using the builtin slices and maps packages instead of the recently deleted future package has lowered coverage, this improves it.

Which issue does this change relate to?

N/A

Contribution checklist.

• [x] I have read and understood the CONTRIBUTING guidelines
• [x] My code is formatted (make check)
• [x] I have run tests (make test)
• [x] All commits in my PR conform to the commit hygiene section
• [x] I have added relevant tests
• [x] I have not added any dependencies

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.03%. Comparing base (1b798cf) to head (8da6157).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v2 #114 +/- ## ========================================== + Coverage 93.78% 95.03% +1.24% ========================================== Files 16 16 Lines 322 322 ========================================== + Hits 302 306 +4 + Misses 10 8 -2 + Partials 10 8 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.