Make license headers optional for test directories.

Test data has some case-by-case exceptions for the license header
requirements. The blocking test should only enforce header correctness,
leaving the choice of which files need headers for individual reviewers
to make manually.

Change-Id: I1cec04e8763c6102b485638058df37d9093ed85a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>