Shrink FieldRefRandomTest
Shrink FieldRefRandomTest

Summary: This test takes up more than 90% of the time of running `buck test thrift/lib/cpp2/...`, which is a substantial devX hit for only probabilistic coverage. Reduce substantially in dev mode and less substantially in opt mode to better balance the tradeoff.

Reviewed By: vitaut

Differential Revision: D24787463

fbshipit-source-id: c0201add810b9cfe6675faedae28ff3640bca36c

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift