add hnsw unit test for PR 3840 (#3849)

Summary:
Pull Request resolved: #3849

#3845

Add unit tests for helper search utilities for HNSW. These utility functions live inside an anonymous namespace and each has a reference version gated behind a const bool, I refactored them so the reference version is a flag for the function which defaults to false.

If we are concerned about the performance overhead of the extra if branching (whether to use reference version or not) inside these utility functions, I'm happy to lift out the reference versions to their own functions inside the unit test

Reviewed By: junjieqi

Differential Revision: D62510014

fbshipit-source-id: b92ed4db69d125c7830da93946f1c2374fb87b08

Author: Mengdi Lin