Fix shardsplitting test
Fix shardsplitting test

Summary:
I changed the logic to send **all** operations to one particular shard split.
A test for delete was flaky because we were always returning result from primary.

Reviewed By: jmswen

Differential Revision: D4141741

fbshipit-source-id: fc0fbcdf0ceb9345573901919e094bdf73420819

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter