[libc++] lost_wakeup.pass.cpp test case for atomic_wait is flaky

The lost_wakeup.pass.cpp test case added in #161086 exhibits frequent hangs.

From https://github.com/llvm/llvm-project/issues/161086#issuecomment-3799349669:

> I'm not sure what's happening but I ran the test manually a number of times now, and at first it took like 1-2 seconds on every run. Then suddenly it started running for like 100 seconds. It seems to be affected quite a bit by the load of the machine it is running on, and the load on our servers vary a lot.
>
> I've disabled the testcase downstream because it fails too often for us, but thought I'd mention here too that we see flakiness related to it, probably related to the machine load.
>
> We're running on linux, RHEL 8 machines.