Unit test for the "Mixin" Sink

[dariomannu]

Create a unit test suite for the Mixin sink.

Please make sure test cases are in BDD styles matching the other sinks.

[TanmayRanaware]

Hi @dariomannu i would like to work on this issue. Can you assign this to me?

[dariomannu]

Hi @TanmayRanaware, it's all yours!
Please give a shout if you need any assistance, if you're stuck, of if you find anything unclear.

Also, if you like Rimmel, feel free to drop a star and help spread the word. We're all growing it together!

[TanmayRanaware]

hey @dariomannu i have added the PR #39 . Can you review and merge. Let me knwo if you need any clarification

[TanmayRanaware]

@dariomannu fixed the comments. Can you review and merge.