Add tests/data/* to .agignore

Summary:
Removes some noise from the results when grepping the source code with
'ag'.

References:

- The Silver Searcher (ag)
    http://geoff.greer.fm/ag/

- Vim plugin for the_silver_searcher, 'ag'
    https://github.com/rking/ag.vim

Test Plan: None

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D551