fix flaky test from uninitialized std::atomic

fix flaky test from uninitialized std::atomic

Summary:
std::atomic<bool> default constructor doesn't initialize it to
false, so the flag to end the LifoSem.multi_try_wait test was sometimes
set too early.  This diff fixes that, makes the test more deterministic,
and also fixes a couple of other benign uninitialized values (so that
they won't be used as prototypes for places where it does matter).

Test Plan: unit tests

Reviewed By: mpawlowski@fb.com

Subscribers: njormrod, lovro

FB internal diff: D1604508

Tasks: 5336837