Simplify users API QA spec
After reading gitlab-qa#251 (closed) I took a look at this API spec for the first time. As this is the first test of its kind in QA, I think it's important to establish best practices for verifying the results from the API, so I made a minor change that makes the test a bit cleaner and more robust. For example, .first has bitten us badly in the past and I want to avoid it at all costs.