Rather than bodging the test to make it more robust, fix the functions
in the Threading layer to behave the same as they do on other platforms,
i.e. to guarantee that they always wait *at least* the amount of time
you asked for.
rdar://100236038
Windows' behaviour wrt `Sleep` family functions can be odd. Apparently,
if you specify a time lower than a system tick, they can return early.
This makes the test flaky when it shouldn't be. To fix it, we use the
Multimedia functions to adjust the system tick count as low as we can
get it.
rdar://100236038
