Move spec-only helper class out of the rack namespace

[Earlopain]

Rack may add Rack::MockResponse::Cookie itself: rack/rack#2327

[grape-bot]

	1 Warning
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

	1 Message
📖	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Here's an example of a CHANGELOG.md entry:

* [#2565](https://github.com/ruby-grape/grape/pull/2565): Move spec-only helper class out of the rack namespace - [@Earlopain](https://github.com/Earlopain).

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