Test exhaust cursors exhaustively. Cover the same four cases as PyMongo:

https://github.com/mongodb/mongo-python-driver/blob/master/test/test_client.py#L844

... but do it for both pooled and single-threaded mongoc_client_t's, and use mock_server_t instead of a live server. So the matrix is:

• Network error / server error
• Error during initial query / getmore
• Pooled / single-threaded client