_rangeDeleterTaskExecutor is currently initialized by ShardingState::_initializeRangeDeleterTaskExecutor().

We want to make this a public function, and have it take "NetworkInterface* netPtr" as a parameter with which to initialize the task executor.

This shiny new public function will get called above this code: https://github.com/mongodb/mongo/blob/master/src/mongo/db/db.cpp#L712-L738, which calls the regular sharding initialization code.

Now in testing we can set up a task executor with a mock network interface. In ShardingStateTest::setUp, we want to call our shiny new function above this code https://github.com/mongodb/mongo/blob/master/src/mongo/db/s/sharding_state_test.cpp#L135-L139

And finally we want to test the task executor, so make a test to give it a silly task – e.g. pass in a variable by reference that gets set to some value --, wait for it to finish, check it did what you wanted.