Feat/alex wan/tests

Because this needs to be executed after `post '/user'`, we have to place this in the same test method.
Unit tests are meant to be done in isolation. As a result, a new instance of this test class is created for each method call, creating a new uuid each time.

Additionally, id is passed as a path variable, not as a HTTP parameter.
So change to `put "/user/id/#{@user_uuid}"`