Add missing < 0 tests for constexpr_strcmp
Add missing < 0 tests for constexpr_strcmp

Summary: [Folly] Add missing `< 0` tests for `constexpr_strcmp` to verify that the implementation is symmetric.

Reviewed By: Mizuchi

Differential Revision: D23482134

fbshipit-source-id: d3cb1fe8ce4690a34a12620eb57e7a8ff57ad8a1

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