Add tests for reduce

[cddr]

Add tests for:

• reduce

[CLAassistant]

All committers have signed the CLA.

[frenchy64]

There's a few more edge cases documented in the reduce docstring if you're interested. Also reduced.

[jeaye]

Thanks for the PR, Andy! reduced is its own var, so that should go into its own file. We don't need to worry about it for this PR. @cddr are you interested in expanding on this for the edge cases which @frenchy64 mentioned?

[cddr]

Ah sorry, notifications were not getting through. Yeah I'll do those this weekend.

[jeaye]

Thanks! Have you added everything you'd like to add, at this point?

[cddr]

For reduce yes. Might do some other functions in the future.

[jeaye]

Sounds great. Thanks for the help!