Most of the integration testing of aggregation in a sharded cluster happens in jstests/aggregation/testshard1.js, but there are many other tests that are not stressing any logic in a sharded cluster, leading to bugs like SERVER-25419

We should create a passthrough suite that runs the aggregation tests against a sharded cluster, or move the agg tests to be part of the 'core' suite and leverage those existing passthroughs.

The mongos does very little logic if the collection isn't sharded, so if we add a new passthrough, it should implicitly shard every collection to increase coverage.