Add a simple test for hh_format.

Summary: This will be used to verify my upcoming relative-path
refactoring work.

It would be nice to generalize verify.py to handle hh_format, or at
least to loop over all the files in this directory... but I don't
remember how to write bash for loops off the top of my head.

Reviewed By: @elgenie

Differential Revision: D1645431

Signature: t1:1645431:1414552220:e9ae6735b0615cae4dd142d8d625d8c3bb212764