core (or passthrough) tests are those that do not make assumptions of the cluster topology. For instance:
 - they can be executed on a replica set or a sharded cluster.
 - they do not make any assumption about the config server being embedded or dedicated.
 - they do not make any assumption on the number of replica set nodes.

These types of tests are very useful because we can run them with different cluster topology/configurations and ensure that everything works as expected on all those different configurations.

Currently, we do not have any core tests for commands that can be executed exclusively in sharded cluster (e.g. shardCollection, refineShardKey, resharding etc...). Usually for those commands we rely on sharding tests living in the "jstests/sharding" folder. These tests explicitly spawn a sharded cluster, using the ShardingTest JS fixture, thus we only execute them in one cluster configuration that is the one specified by the test itself.

The goal of this ticket is to introduce a new class of sharding core tests that can be executed in several passthrough suites.

Those tests will be placed under "jstests/core_sharding" folder.