Let test_once be noexcept
Let test_once be noexcept

Summary: [Folly] Let `test_once` be `noexcept` since it is required not to fail.

Reviewed By: andriigrynenko, luciang

Differential Revision: D18742588

fbshipit-source-id: fda78057d10fca1521ee6335f7c7758d05c43b88

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly