Avoid keeping trx stream open in unit tests

[Youssef1313]

TrxTests works by creating a MemoryStream, and mocking NewFileStream to return that instance. Then later, the tests tries to use that stream. For that reason, tests are passing a flag to prevent production code form disposing the stream.

This PR cleans up product code, but makes tests more verbose. Now, product code will always dispose, without any flag that's used by tests. However, the "fake" stream created by tests will create the XDocument that the test needs when Dispose is called. That way, we don't need the stream anymore and we use the XDocument.

@Evangelink How do you feel about this change? I generally don't like when there is a code path in product code that's test only and prefer more ugly tests but cleaner product code. That's a personal taste though.