test user creation, deletion, retrieval, and listing

[stvnjacobs]

I came to report an issue retrieving users, but found @patthiel had already opened an issue for it (https://github.com/linode/linode-cli/issues/42).

I didn't look into the details of why it is failing. It seems like @Dorthu had a good idea of what was going on. Thought I could help and write some tests for it. Taken almost 1:1 from tags.bats, but they do exemplify what I was going to report.

p.s. no clue of this localhost email is accepted by the API. The warning when trying to test it locally about deleting all my things scared me, so I'm letting CI here do it for me.