Make clock tests less flaky

[jkuszmaul]

Currently the tests in util/clock_test.cc tend to be flaky because they they rely on the host machine actually executing the sleeps in a timely manner and not experiencing delays elsewhere.
There's a few things that should probably be done:

• Separate out testing basic code functionality (does the code behave sanely) and performance testing (when running an X hz loop, how accurately do we hit the timing)
• Adjust test environment to better match real applications (e.g., run processes at higher priorities)
• Figure out good ways of measuring performance (e.g., currently we require every single of 100 iterations to be within some tolerance for time accuracy; maybe up that to 1000s of iterations and check that some percentage match our tolerances)