Updated tests to reflect the use of RequestManagerRestSharp over RequestManager.

[cofl]

The use of RequestManagerRestSharp in SnipeItApi was causing the tests CheckApiTokenAndUrl_SetAuthorizationHeader_SetCorrectly and CheckApiTokenAndUrl_SetHttpClientBaseAddress_SetCorrectly to fail with NullReferenceExceptions.

I updated the tests to reflect the use of the new class, and added internals visibility on the main assembly to the test assembly, to limit how much reflection we need to do.