Yarn: Add tests for `yarn owner`

[voxsim]

yarn owner has not tests at all and we should at least cover:

• add
• rm
• list/ls

[peterschussheim]

@voxsim Can I take this one?

[voxsim]

@peterschussheim yep go ahead!

[peterschussheim]

Great! Are there any examples you know would be a good starting point?

[voxsim]

@peterschussheim nope, you can take inspiration from the other tests in the test forlder (for example https://github.com/yarnpkg/yarn/blob/master/__tests__/commands/licenses.js)

[rubycut]

@BYK , owner command seems to be broken, it doesn't return anything. I am trying to fix it, but I am now getting 1 maintainer @Daniel15 , but if I try to run npm owner list yarn, I get six maintainers., so which of these responses are correct?