tests/core_timing: Remove pragma optimize(off)

I made a review comment about this in the PR that this was introduced
in (#3955, commit 71c47792), but it
seems to have been missed.

We shouldn't be using this pragma here because it's MSVC specific. This
causes warnings on other compilers.

The test it's surrounding is *extremely* dubious, but for the sake of
silencing warnings on other compilers, we can mark "placebo" as volatile
and be on with it.