Add support for exploding MarkedSource in Go indexer tests.
Add support for exploding MarkedSource in Go indexer tests.

Summary:
This change doesn't add any "code" facts to the Go graph (yet), but does add a
parameter to the verifier test rules to allow such facts to be tested.

Reviewers: #core_team, jrtom

Reviewed By: #core_team, jrtom

Subscribers: #core_team

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D1444

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