This ticket is responsible for adding test coverage around spilling to disk for the new exact top-n accumulators. In particular:

• Add tests to verify that spilling works correctly for the exact top-n accumulators in jstests/aggregation/spill_to_disk.js
• Add tests to verify that the 'internalQueryMaxNAccumulatorBytes' memory limit is obeyed in jstests/noPassthrough/agg_configurable_memory_limits.js (that is, if this configured to a low value, any top-n operation will fail).

Additionally, we should consider updating the error messages for the various 'ExceededMemoryLimit' uasserts added as part of this project. Specifically, they should clarify that an error was raised because too much memory was used and spilling would not help.