[LSan] Mark create_thread_leak.cpp as UNSUPPORTED: darwin.

It started to fail in a flaky manner a few days ago on GreenDragon buildbots
(i.e. x86_64-darwin). I didn't track down the root cause but LSan isn't
actually supported on darwin anyway, so UNSUPPORTED seems appropriate.

Prior art: 3ff080b5.

(cherry picked from commit 0a3519d5)