campfire: Add 'go_test_args' property for go_test rules.
campfire: Add 'go_test_args' property for go_test rules.

Summary:
This permits a test to set runtime flags on the test binary.
The case I really care about is -test.cpu, for testing concurrency.

Reviewers: schroederc

Reviewed By: schroederc

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D47

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