[TASK] Refactor trustline test code

Refactor trustline test code to reduce duplication and check response body before status code and headers
The method might be misleading, can be clearer. The logic doesn't just check the request, it executes it and tests different parts of the response. After thinking through a couple options, I would go for the simple `testRequest`. To keep it clear where this function is called I would add `get` or `post` to the options object that is passed in.

For consistency, switch the order of the `done` and `options` arguments, put the callback last.