`test_clock_sync` seems flaky

This is similar to [[watchman/issues/1100] `test_sinceRelativeRoot` seems flaky](https://github.com/facebook/watchman/issues/1100)

I'm trying to build from source, and I keep getting flaky results on https://github.com/facebook/watchman/blob/6b0361c3464efcb5b7db11937f0bd9f84c865914/watchman/integration/test_clock.py#L29

While debugging, I added some log lines, and it immediately stopped flaking ([Heisenbug](https://en.wikipedia.org/wiki/Heisenbug) flag!)

I haven't fully looked into the implementation to understand this 100%, but could it be that:

https://github.com/facebook/watchman/blob/6b0361c3464efcb5b7db11937f0bd9f84c865914/watchman/integration/test_clock.py#L21-L27

can flake if not enough time passes between the assignment to `clock1` and the `clock2`?