Bug 985902 - expose fxaccounts/MOZ_SERVICES_FXACCOUNTS to mozinfo and only run test_hawkclient when enabled. r=ted,a=test-only

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extra : transplant_source : %0E%B6%ED%1D%C6%91%DB%99I%95_%00%19r%24%D6%AB%1C%3E%F9