As of SERVER-40381 we'll be using this for more than just testing, so its name should indicate it as such.

As DocumentSourceMock includes testing focused characteristics, like construction of an ExpressionContextForTest, we should consider doing the following:
1) Rename DocumentSourceMock to DocumentSourceQueue
2) Move DocumentSourceQueue into the 'pipeline' library, and reference from the 'update' library (removing the document_source_mock reference).
3) Creating DocumentSourceMock as a subclass of DocumentSourceQueue, pulling the testing aspects out of DocumentSourceQueue into the new DocumentSourceMock.