Increase test coverage

Some more specs and code removal to increase test coverage.
The test only makes sense because `GlobalVars#user_vars` looks like this:

```ruby
        def user_vars
          if cop_config['AllowedVariables']
            cop_config['AllowedVariables'].map(&:to_sym)
          else
            []
          end
        end
```
In real execution, the `AllowedVariables` parameter can only be `nil` if the user overrides the default value (which is `[]`) by setting

```yaml
Style/GlobalVars:
  AllowedVariables: ~
```
I only learned recently that `nil`can be represented in YAML, and an override like this is something that we don't handle for most cops. I think it's better to not handle `nil` for any configuration parameters with a non-nil default value. It follows that tests like this one should be removed or changed.