codegraph: unit test for the link between def and use of xhp attribute
codegraph: unit test for the link between def and use of xhp attribute

Summary: Testing the new feature jiao recently added to codegraph.

Test Plan: make test

Reviewers: jiaol, pieter, mathieubaudet

Reviewed By: jiaol

Differential Revision: https://phabricator.fb.com/D849435

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