ExpressionContextForTest owns a service context. DocumentSourceMock will create an ExpressionContextForTest in its create() methods. This means that each time a DocumentSourceMock is created, we go through all of the work of creating an entire service context.

At the same time, the AggregationContextFixture inherits from ServiceContextFixture, which already makes a global ServiceContext available. This means that many of the aggregation stage unit tests actually have (at least) two service contexts: one globally and another as part of the ExpressionContextForTest.

While there is no immediate bug here, this was responsible for:

SERVER-41053

And also caused a lot of confusion while doing investigations for

SERVER-41042 and SERVER-39447

The ownership hierarchy should probably changed so that there's just one ServiceContext for the lifetime of the test which is threaded through ExpressionContextForTest.