Test is unreliable on macOS 10.13. The lower-bound isn't central to the
purpose of the test, so just relax it.
ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
> We don't guarantee that a X ms timer is triggered during Y ms sleep
> for any X<Y, though I would expect the load to be really bad for this
> to happen with X=10ms, Y=40ms.
related: #6829